dart2js: turn on sync-async by default

Closes https://github.com/dart-lang/sdk/issues/32869

Change-Id: I79b2d02d13adccdfbbd3a78158805244aceff7de
Reviewed-on: https://dart-review.googlesource.com/52061
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Stephen Adams <sra@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

CHANGELOG.md

@@ -28,6 +28,11 @@
 
 ### Tool Changes
 
+#### dart2js
+
+* `async` functions now start synchronously. This will be the default behavior
+  across all tools. To temporarily opt-out, use the `--no-sync-async` flag.
+
 #### Pub
 
 * Fix another bug where the version solver could crash when resolving a conflict

pkg/compiler/lib/src/commandline_options.dart

@@ -95,11 +95,19 @@ class Flags {
   // https://gist.github.com/eernstg/4353d7b4f669745bed3a5423e04a453c.
   static const String genericMethodSyntax = '--generic-method-syntax';
 
-  // Starts `async` functions synchronously.
+  // Deprecated. This flag is no longer in use for dart2js, but we are keeping
+  // it around for a while longer until all other tools deprecate the same flag.
   //
-  // This is the Dart 2.0 behavior. This flag is only used during the migration.
+  // It was used to start `async` functions synchronously, but now dart2js
+  // switched on this behavior by default.
+  // TODO(sigmund): delete once this is on by default on all of our tools.
   static const String syncAsync = '--sync-async';
 
+  // Starts `async` functions asynchronously.
+  //
+  // This is the old Dart 1.0 behavior. Only used during the migration.
+  static const String noSyncAsync = '--no-sync-async';
+
   // Initializing-formal access is enabled by default and cannot be disabled.
   // For backward compatibility the option is still accepted, but it is ignored.
   static const String initializingFormalAccess = '--initializing-formal-access';

pkg/compiler/lib/src/dart2js.dart

@@ -317,7 +317,8 @@ Future<api.CompilationResult> compile(List<String> argv,
     new OptionHandler(Flags.allowMockCompilation, ignoreOption),
     new OptionHandler(Flags.fastStartup, passThrough),
     new OptionHandler(Flags.genericMethodSyntax, ignoreOption),
-    new OptionHandler(Flags.syncAsync, passThrough),
+    new OptionHandler(Flags.syncAsync, ignoreOption),
+    new OptionHandler(Flags.noSyncAsync, passThrough),
     new OptionHandler(Flags.initializingFormalAccess, ignoreOption),
     new OptionHandler(Flags.minify, passThrough),
     new OptionHandler(Flags.preserveUris, ignoreOption),

pkg/compiler/lib/src/options.dart

@@ -348,7 +348,7 @@ class CompilerOptions implements DiagnosticOptions {
       ..useMultiSourceInfo = _hasOption(options, Flags.useMultiSourceInfo)
       ..useNewSourceInfo = _hasOption(options, Flags.useNewSourceInfo)
       ..useStartupEmitter = _hasOption(options, Flags.fastStartup)
-      ..startAsyncSynchronously = _hasOption(options, Flags.syncAsync)
+      ..startAsyncSynchronously = !_hasOption(options, Flags.noSyncAsync)
       ..verbose = _hasOption(options, Flags.verbose);
   }
 

runtime/observatory/tests/observatory_ui/mocks/repositories/flag.dart

@@ -9,6 +9,7 @@ class FlagsRepositoryMock implements M.FlagsRepository {
   bool isListInvoked = false;
 
   Future<Iterable<M.Flag>> list() async {
+    await null;
     isListInvoked = true;
     return _list;
   }

tests/co19/co19-dart2js.status

@@ -17,6 +17,7 @@ Language/Expressions/Function_Invocation/async_cleanup_t06: Skip # https://githu
 Language/Expressions/Function_Invocation/async_cleanup_t08: Skip # https://github.com/dart-lang/sdk/issues/28873
 Language/Expressions/Function_Invocation/async_generator_invokation_t08: Skip # Issue 25967
 Language/Expressions/Function_Invocation/async_generator_invokation_t10: Skip # Issue 25967
+Language/Expressions/Function_Invocation/async_invokation_t02: RuntimeError # sync-async is on by default.
 Language/Expressions/Function_Invocation/async_invokation_t04: RuntimeError, Pass # co19 issue 57
 Language/Expressions/Instance_Creation/New/evaluation_t19: RuntimeError # Please triage this failure
 Language/Expressions/Instance_Creation/New/evaluation_t20: RuntimeError # Please triage this failure

tests/compiler/dart2js/sourcemaps/stacktrace/throw_in_async.dart

@@ -11,5 +11,6 @@ main() {
 
 @NoInline()
 test() async {
+  await null;
   /*1:test*/ throw '>ExceptionMarker<';
 }

tests/compiler/dart2js/sourcemaps/stacktrace/throw_in_awaited_async.dart

@@ -16,5 +16,6 @@ test1() async {
 
 @NoInline()
 test2() async {
+  await null;
   /*1:test2*/ throw '>ExceptionMarker<';
 }

tests/compiler/dart2js/sourcemaps/stacktrace/throw_in_constructor_from_async.dart

@@ -10,6 +10,7 @@ main() {
 }
 
 test() async {
+  await null;
   // ignore: UNUSED_LOCAL_VARIABLE
   var c = new /*1:test*/ Class();
 }

tests/compiler/dart2js/sourcemaps/stacktrace/throw_in_top_level_method_from_async.dart

@@ -10,6 +10,7 @@ main() {
 
 @NoInline()
 test1() async {
+  await null;
   /*1:test1*/ test2();
 }
 

tests/language/language_dart2js.status

@@ -5,8 +5,11 @@
 [ $compiler != dart2analyzer ]
 switch_case_warn_test: Skip # Analyzer only, see language_analyzer2.status
 
-# VM specific tests that should not be run by dart2js.
 [ $compiler == dart2js ]
+async_await_test: RuntimeError # sync-async is on by default
+asyncstar_throw_in_catch_test: RuntimeError # sync-async is on by default
+await_nonfuture_test: RuntimeError # sync-async is on by default
+await_not_started_immediately_test: RuntimeError # sync-async is on by default
 full_stacktrace1_test: Pass, RuntimeError # Issue 12698
 full_stacktrace2_test: Pass, RuntimeError # Issue 12698
 full_stacktrace3_test: Pass, RuntimeError # Issue 12698

tests/lib/lib.status

@@ -15,6 +15,7 @@ mirrors/*: Skip # Issue 27929: Triage
 *: Skip
 
 [ $compiler == dart2js ]
+async/async_await_sync_completer_test: RuntimeError # sync-async is on by default.
 async/schedule_microtask6_test: RuntimeError # global error handling is not supported. Issue 5958
 convert/base64_test/01: Fail, OK # Uses bit-wise operations to detect invalid values. Some large invalid values accepted by dart2js.
 convert/chunked_conversion_utf88_test: Slow, Pass