Fixed the conditions of an assert causing the debug VM to crash when trying to load a library from an invalid path. Created copy of a previous deferred_import_t02.dart test from co19 as a regression test until deferred_import_t02.dart is reverted to the previous test.

Fixes #27201

R=zra@google.com

Review URL: https://codereview.chromium.org/2537253002 .

runtime/vm/object.cc

@@ -11008,8 +11008,8 @@ bool LibraryPrefix::LoadLibrary() const {
       Exceptions::PropagateError(Error::Cast(obj));
     }
   } else {
-    // Another load request is in flight.
-    ASSERT(deferred_lib.LoadRequested());
+    // Another load request is in flight or previously failed.
+    ASSERT(deferred_lib.LoadRequested() || deferred_lib.LoadFailed());
   }
   return false;  // Load request not yet completed.
 }

tests/language/vm/regress_27201_lib.dart

@@ -0,0 +1,20 @@
+// Copyright (c) 2016, 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.
+
+library import_lib;
+
+final foo = 1;
+var someVar = 3;
+var _privateVar;
+
+int get someGetter => 2;
+
+void set someSetter(int val) {}
+
+int someFunc() => 0;
+
+class SomeClass {
+}
+
+typedef int Func(Object a);

tests/language/vm/regress_27201_test.dart

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2016, 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.
+ */
+import "dart:async";
+import "package:expect/expect.dart";
+import "regress_27201_lib.dart" deferred as p;
+import "regress_27201_bad_lib_path.dart" deferred as q;
+
+test_loaded() {
+  try {
+    p.someFunc();
+  } catch (e) {
+    Expect.fail("Should not be here");
+  }
+  try {
+    p.someGetter;
+  } catch (e) {
+    Expect.fail("Should not be here");
+  }
+  try {
+    p.someSetter = 1;
+  } catch (e) {
+    Expect.fail("Should not be here");
+  }
+  try {
+    p.Func;
+  } catch (e) {
+    Expect.fail("Should not be here");
+  }
+  try {
+    Expect.isTrue(p.loadLibrary() is Future);
+  } catch (e) {
+    Expect.fail("Should not be here");
+  }
+}
+
+
+main() {
+  p.loadLibrary().then((v) {
+    test_loaded();
+  },
+  onError: (e) {
+    Expect.fail("Should have loaded library!");
+  });
+
+  // Ensure bad library import is handled correctly.
+  q.loadLibrary().then((v) {
+    Expect.fail("Should have failed");
+  },
+  onError: (e) {
+    Expect.throws(() => q.x);
+  });
+}
+