diff --git a/pkg/compiler/lib/src/js_backend/resolution_listener.dart b/pkg/compiler/lib/src/js_backend/resolution_listener.dart
index 289ae4290c5..118318285a4 100644
--- a/pkg/compiler/lib/src/js_backend/resolution_listener.dart
+++ b/pkg/compiler/lib/src/js_backend/resolution_listener.dart
@@ -312,26 +312,14 @@ class ResolutionEnqueuerListener extends EnqueuerListener {
     }
 
     // Enable isolate support if we start using something from the isolate
-    // library, or timers for the async library.  We exclude constant fields,
-    // which are ending here because their initializing expression is
-    // compiled.
-    LibraryEntity library = member.library;
-    if (!_backendUsage.isIsolateInUse && !(member.isField && member.isConst)) {
-      Uri uri = library.canonicalUri;
-      if (uri == Uris.dart_isolate) {
-        _backendUsage.isIsolateInUse = true;
-        worldImpact
-            .addImpact(_enableIsolateSupport(_elementEnvironment.mainFunction));
-      } else if (uri == Uris.dart_async) {
-        if (member.name == '_createTimer' ||
-            member.name == '_createPeriodicTimer') {
-          // The [:Timer:] class uses the event queue of the isolate
-          // library, so we make sure that event queue is generated.
-          _backendUsage.isIsolateInUse = true;
-          worldImpact.addImpact(
-              _enableIsolateSupport(_elementEnvironment.mainFunction));
-        }
-      }
+    // library.  We exclude constant fields, which are ending here because their
+    // initializing expression is compiled.
+    if (!_backendUsage.isIsolateInUse &&
+        !(member.isField && member.isConst) &&
+        member.library.canonicalUri == Uris.dart_isolate) {
+      _backendUsage.isIsolateInUse = true;
+      worldImpact
+          .addImpact(_enableIsolateSupport(_elementEnvironment.mainFunction));
     }
 
     if (member.isGetter && member.name == Identifiers.runtimeType_) {
@@ -437,6 +425,10 @@ class ResolutionEnqueuerListener extends EnqueuerListener {
       _interceptorData.addInterceptorsForNativeClassMembers(cls);
     } else if (cls == _commonElements.jsIndexingBehaviorInterface) {
       _registerBackendImpact(impactBuilder, _impacts.jsIndexingBehavior);
+    } else if (cls.library.canonicalUri == Uris.dart_isolate) {
+      _backendUsage.isIsolateInUse = true;
+      impactBuilder
+          .addImpact(_enableIsolateSupport(_elementEnvironment.mainFunction));
     }
 
     _customElementsAnalysis.registerInstantiatedClass(cls);
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
index 86ef3ecc6b6..1b9ccc25fca 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
@@ -72,20 +72,15 @@ _callInIsolate(_IsolateContext isolate, Function function) {
 ///
 /// These functions only has to be called for code that can be run from a
 /// worker-isolate (so not for general dom operations).
-enterJsAsync() {
-  _globalState.topEventLoop._activeJsAsyncCount++;
-}
+enterJsAsync() {}
 
 /// Marks leaving a javascript async operation.
 ///
 /// See [enterJsAsync].
-leaveJsAsync() {
-  _globalState.topEventLoop._activeJsAsyncCount--;
-  assert(_globalState.topEventLoop._activeJsAsyncCount >= 0);
-}
+leaveJsAsync() {}
 
 /// Returns true if we are currently in a worker context.
-bool isWorker() => _globalState.isWorker;
+bool isWorker() => globalWindow == null && globalPostMessageDefined;
 
 /**
  * Called by the compiler to fetch the current isolate context.
@@ -787,7 +782,7 @@ class IsolateNatives {
       return JS('String', 'String(#.src)', currentScript);
     }
     // A worker has no script tag - so get an url from a stack-trace.
-    if (_globalState.isWorker) return computeThisScriptFromTrace();
+    if (isWorker()) return computeThisScriptFromTrace();
     // An isolate that doesn't support workers, but doesn't have a
     // currentScript either. This is most likely a Chrome extension.
     return null;
@@ -1372,31 +1367,11 @@ class ReceivePortImpl extends Stream implements ReceivePort {
 
 class TimerImpl implements Timer {
   final bool _once;
-  bool _inEventLoop = false;
   int _handle;
   int _tick = 0;
 
   TimerImpl(int milliseconds, void callback()) : _once = true {
-    if (milliseconds == 0 && (!hasTimer() || _globalState.isWorker)) {
-      void internalCallback() {
-        _handle = null;
-        callback();
-      }
-
-      // Setting _handle to something different from null indicates that the
-      // callback has not been run. Hence, the choice of 1 is arbitrary.
-      _handle = 1;
-
-      // This makes a dependency between the async library and the
-      // event loop of the isolate library. The compiler makes sure
-      // that the event loop is compiled if [Timer] is used.
-      // TODO(7907): In case of web workers, we need to use the event
-      // loop instead of setTimeout, to make sure the futures get executed in
-      // order.
-      _globalState.topEventLoop
-          .enqueue(_globalState.currentContext, internalCallback, 'timer');
-      _inEventLoop = true;
-    } else if (hasTimer()) {
+    if (hasTimer()) {
       void internalCallback() {
         _handle = null;
         leaveJsAsync();
@@ -1409,8 +1384,7 @@ class TimerImpl implements Timer {
       _handle = JS('int', 'self.setTimeout(#, #)',
           convertDartClosureToJS(internalCallback, 0), milliseconds);
     } else {
-      assert(milliseconds > 0);
-      throw new UnsupportedError('Timer greater than 0.');
+      throw new UnsupportedError('`setTimeout()` not found.');
     }
   }
 
@@ -1443,9 +1417,6 @@ class TimerImpl implements Timer {
 
   void cancel() {
     if (hasTimer()) {
-      if (_inEventLoop) {
-        throw new UnsupportedError('Timer in event loop cannot be canceled.');
-      }
       if (_handle == null) return;
       leaveJsAsync();
       if (_once) {
diff --git a/tests/co19/co19-dart2js.status b/tests/co19/co19-dart2js.status
index ceb259275c6..d8f09d78344 100644
--- a/tests/co19/co19-dart2js.status
+++ b/tests/co19/co19-dart2js.status
@@ -188,7 +188,8 @@ LibTest/isolate/Isolate/spawnUri_A01_t06: RuntimeError # Dart issue 15974
 LibTest/isolate/Isolate/spawnUri_A01_t07: RuntimeError # Dart issue 15974
 LibTest/isolate/Isolate/spawnUri_A02_t01: RuntimeError, Pass # Dart issue 15617
 LibTest/isolate/Isolate/spawn_A02_t02: RuntimeError, Pass # Dart issue 15617
-LibTest/isolate/Isolate/spawn_A04_t01: RuntimeError # Dart issue 15974
+LibTest/isolate/Isolate/spawn_A04_t01: SkipByDesign
+LibTest/isolate/Isolate/spawn_A04_t02: SkipByDesign
 LibTest/isolate/Isolate/spawn_A06_t06: Skip # Times out. Please triage this failure.
 LibTest/math/MutableRectangle/boundingBox_A01_t01: RuntimeError, Pass # co19-roll r706: Please triage this failure.
 LibTest/math/Rectangle/boundingBox_A01_t01: RuntimeError, Pass # co19-roll r706: Please triage this failure.
diff --git a/tests/compiler/dart2js_extra/dart2js_extra.status b/tests/compiler/dart2js_extra/dart2js_extra.status
index 5dce5f5eeb5..c44b6e22d0f 100644
--- a/tests/compiler/dart2js_extra/dart2js_extra.status
+++ b/tests/compiler/dart2js_extra/dart2js_extra.status
@@ -43,7 +43,6 @@ no_such_method_mirrors_test: Pass, Slow # TODO(kasperl): Please triage.
 
 [ $compiler == dart2js && $runtime == d8 && $fasta ]
 deferred_fail_and_retry_test: RuntimeError # Uses XHR in dart:html
-deferred_fail_and_retry_worker_test: Fail # Uses XHR in dart:html
 unconditional_dartio_import_test: RuntimeError # Uses dart:io
 
 [ $compiler == dart2js && $runtime == ff && $system == windows ]
@@ -52,9 +51,6 @@ consistent_index_error_string_test: Pass, Slow # Issue 25940
 [ $compiler == dart2js && $runtime == none ]
 *: Fail, Pass # TODO(ahe): Triage these tests.
 
-[ $compiler == dart2js && $runtime == safari ]
-deferred_fail_and_retry_worker_test: Timeout # Issue 22106
-
 [ $compiler == dart2js && $checked ]
 variable_type_test/01: Fail, OK
 variable_type_test/03: Fail, OK
@@ -77,7 +73,6 @@ closure_signature_unneeded_test: RuntimeError # Too eager signature generation.
 [ $compiler == dart2js && $csp ]
 deferred_custom_loader_test: SkipByDesign # Issue 25683
 deferred_fail_and_retry_test: SkipByDesign # Uses eval to simulate failed loading.
-deferred_fail_and_retry_worker_test: SkipByDesign # Uses eval to simulate failed loading.
 js_interop_optional_arg_test: RuntimeError # Issue 31082
 js_interop_test: RuntimeError # Issue 31082
 
diff --git a/tests/compiler/dart2js_extra/deferred_fail_and_retry_worker_test.dart b/tests/compiler/dart2js_extra/deferred_fail_and_retry_worker_test.dart
deleted file mode 100644
index dffa7391403..00000000000
--- a/tests/compiler/dart2js_extra/deferred_fail_and_retry_worker_test.dart
+++ /dev/null
@@ -1,60 +0,0 @@
-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// Test that when a deferred import from a worker fails to load, it is possible
-// to retry.
-
-import "deferred_fail_and_retry_lib.dart" deferred as lib;
-import "package:expect/expect.dart";
-import "package:async_helper/async_helper.dart";
-import "dart:isolate";
-import "dart:js" as js;
-
-void test(SendPort sendPort) {
-  // Patch XMLHttpRequest to fail on first load.
-  js.context.callMethod("eval", [
-    """
-    oldXMLHttpRequest = XMLHttpRequest;
-    XMLHttpRequest = function() {
-      XMLHttpRequest = oldXMLHttpRequest;
-      var instance = new XMLHttpRequest();
-      this.addEventListener = function(x, y, z) {
-        instance.addEventListener(x, y, z);
-      }
-      this.send = function() {
-        instance.send();
-      }
-      this.open = function(x, uri) {
-        instance.open(x, "non_existing.js");
-      }
-    }
-  """
-  ]);
-  lib.loadLibrary().then((_) {
-    sendPort.send("Library should not have loaded");
-  }, onError: (error) {
-    sendPort.send("failed");
-    lib.loadLibrary().then((_) {
-      sendPort.send(lib.foo());
-    }, onError: (error) {
-      sendPort.send("Library should have loaded this time $error");
-    });
-  });
-}
-
-main() {
-  ReceivePort receivePort = new ReceivePort();
-  asyncStart();
-  bool receivedFailed = false;
-  receivePort.listen((message) {
-    if (!receivedFailed) {
-      Expect.equals("failed", message);
-      receivedFailed = true;
-    } else {
-      Expect.equals("loaded", message);
-      asyncEnd();
-    }
-  });
-  Isolate.spawn(test, receivePort.sendPort);
-}
diff --git a/tests/compiler/dart2js_native/dart2js_native.status b/tests/compiler/dart2js_native/dart2js_native.status
index d3f454e6c2c..45d07d37b64 100644
--- a/tests/compiler/dart2js_native/dart2js_native.status
+++ b/tests/compiler/dart2js_native/dart2js_native.status
@@ -14,9 +14,6 @@ native_no_such_method_exception5_frog_test: SkipByDesign # mirrors not supported
 [ $browser ]
 *: Skip
 
-[ $compiler == dart2js && $runtime == d8 && $system == windows && !$fasta ]
-compute_this_script_test: Skip # Issue 17458
-
 [ $compiler == dart2js && $fasta ]
 native_library_same_name_used_frog_test: CompileTimeError
 subclassing_constructor_1_test: RuntimeError
@@ -24,8 +21,5 @@ subclassing_super_call_test: RuntimeError
 subclassing_super_field_1_test: RuntimeError
 subclassing_super_field_2_test: RuntimeError
 
-[ $compiler == dart2js && $fasta && $host_checked ]
-compute_this_script_test: Crash # 'file:*/pkg/compiler/lib/src/ssa/types.dart': Failed assertion: line 63 pos 12: '!result.isEmpty': is not true.
-
 [ $compiler == dart2js && $minified ]
 optimization_hints_test: RuntimeError, OK # Test relies on unminified names.
diff --git a/tests/html/html.status b/tests/html/html.status
index 632145d466e..be29f941cc1 100644
--- a/tests/html/html.status
+++ b/tests/html/html.status
@@ -26,6 +26,8 @@ webgl_1_test: StaticWarning
 window_nosuchmethod_test: StaticWarning
 
 [ $compiler == dart2js ]
+async_spawnuri_test: SkipByDesign
+async_test: SkipByDesign
 custom/document_register_type_extensions_test/construction: Pass, Timeout # Roll 50 failure
 custom/document_register_type_extensions_test/registration: Pass, Timeout # Roll 50 failure
 custom/entered_left_view_test/shadow_dom: Pass, Timeout # Roll 50 failure
@@ -194,7 +196,6 @@ xhr_test/json: Fail # TODO(dart2js-team): Please triage this failure.
 
 [ $compiler == dart2js && $runtime == d8 && $fasta ]
 async_spawnuri_test: RuntimeError
-async_test: RuntimeError
 audiobuffersourcenode_test: RuntimeError
 audiocontext_test: RuntimeError
 audioelement_test: RuntimeError
diff --git a/tests/isolate/isolate.status b/tests/isolate/isolate.status
index 753bf0c29a5..c5dcc41273a 100644
--- a/tests/isolate/isolate.status
+++ b/tests/isolate/isolate.status
@@ -7,6 +7,7 @@ browser/typed_data_message_test: StaticWarning
 
 [ $compiler == dart2js ]
 browser/issue_12474_test: CompileTimeError # Issue 22529
+deferred_in_isolate2_test: SkipByDesign
 enum_const_test/02: RuntimeError # Issue 21817
 error_at_spawnuri_test: SkipByDesign # Test uses a ".dart" URI.
 error_exit_at_spawnuri_test: SkipByDesign # Test uses a ".dart" URI.
@@ -23,9 +24,11 @@ message3_test/constList_identical: RuntimeError # Issue 21817
 message3_test/constMap: RuntimeError # Issue 21817
 non_fatal_exception_in_timer_callback_test: Skip # Issue 23876
 spawn_uri_exported_main_test: SkipByDesign # Test uses a ".dart" URI.
+spawn_uri_missing_from_isolate_test: SkipByDesign
 spawn_uri_nested_vm_test: SkipByDesign # Test uses a ".dart" URI.
 spawn_uri_vm_test: SkipByDesign # Test uses a ".dart" URI.
 stacktrace_message_test: RuntimeError # Fails to send stacktrace object.
+timer_isolate_test: SkipByDesign
 
 [ $compiler == fasta ]
 browser/compute_this_script_browser_test: CompileTimeError # TODO(ahe): Support dart:html in Fasta.
diff --git a/tests/lib_2/lib_2_dart2js.status b/tests/lib_2/lib_2_dart2js.status
index 6277cb5ee57..5250745d057 100644
--- a/tests/lib_2/lib_2_dart2js.status
+++ b/tests/lib_2/lib_2_dart2js.status
@@ -8,6 +8,8 @@ convert/base64_test/01: Fail, OK # Uses bit-wise operations to detect invalid va
 convert/chunked_conversion_utf88_test: Slow, Pass
 convert/utf85_test: Slow, Pass
 developer/timeline_test: Skip # Not supported
+html/async_spawnuri_test: SkipByDesign
+html/async_test: SkipByDesign
 html/custom/document_register_type_extensions_test/construction: Pass, Timeout # Roll 50 failure
 html/custom/document_register_type_extensions_test/registration: Pass, Timeout # Roll 50 failure
 html/custom/element_upgrade_failure_test: MissingCompileTimeError
@@ -26,6 +28,7 @@ html/svgelement_test/PathElement: Pass, RuntimeError # Roll 50 failure
 html/wrapping_collections_test: SkipByDesign # Testing an issue that is only relevant to Dartium
 html/xhr_test: Pass, Slow
 isolate/browser/issue_12474_test: CompileTimeError # Issue 22529
+isolate/deferred_in_isolate2_test: SkipByDesign
 isolate/enum_const_test/02: RuntimeError # Issue 21817
 isolate/error_at_spawnuri_test: SkipByDesign # Test uses a ".dart" URI.
 isolate/error_exit_at_spawnuri_test: SkipByDesign # Test uses a ".dart" URI.
@@ -42,9 +45,11 @@ isolate/message3_test/constList_identical: RuntimeError # Issue 21817
 isolate/message3_test/constMap: RuntimeError # Issue 21817
 isolate/non_fatal_exception_in_timer_callback_test: Skip # Issue 23876
 isolate/spawn_uri_exported_main_test: SkipByDesign # Test uses a ".dart" URI.
+isolate/spawn_uri_missing_from_isolate_test: SkipByDesign
 isolate/spawn_uri_nested_vm_test: SkipByDesign # Test uses a ".dart" URI.
 isolate/spawn_uri_vm_test: SkipByDesign # Test uses a ".dart" URI.
 isolate/stacktrace_message_test: RuntimeError # Fails to send stacktrace object.
+isolate/timer_isolate_test: SkipByDesign
 math/double_pow_test: RuntimeError
 math/low_test: RuntimeError
 math/random_big_test: RuntimeError # Using bigint seeds for random.