Make dart2js unittests async.

tests/compiler/dart2js/dart_backend_test.dart

 // Copyright (c) 2012, 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 "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart' as leg;
@@ skipping 445 matching lines @@
   new PlaceholderCollector(compiler, new Set<String>(), new DynoMap(compiler))
       ..collect(element);
 
 testLocalFunctionPlaceholder() {
   var src = '''
 main() {
   function localfoo() {}
   localfoo();
 }
 ''';
-  MockCompiler compiler = new MockCompiler(emitJavaScript: false);
-  assert(compiler.backend is DartBackend);
-  compiler.parseScript(src);
-  FunctionElement mainElement = compiler.mainApp.find(leg.Compiler.MAIN);
-  compiler.processQueue(compiler.enqueuer.resolution, mainElement);
-  PlaceholderCollector collector = collectPlaceholders(compiler, mainElement);
-  FunctionExpression mainNode = mainElement.parseNode(compiler);
-  Block body = mainNode.body;
-  FunctionDeclaration functionDeclaration = body.statements.nodes.head;
-  FunctionExpression fooNode = functionDeclaration.function;
-  LocalPlaceholder fooPlaceholder =
-      collector.functionScopes[mainElement].localPlaceholders.first;
-  Expect.isTrue(fooPlaceholder.nodes.contains(fooNode.name));
+  MockCompiler compiler = new MockCompiler.internal(emitJavaScript: false);
+  asyncTest(() => compiler.init().then((_) {
+    assert(compiler.backend is DartBackend);
+    compiler.parseScript(src);
+    FunctionElement mainElement = compiler.mainApp.find(leg.Compiler.MAIN);
+    compiler.processQueue(compiler.enqueuer.resolution, mainElement);
+    PlaceholderCollector collector = collectPlaceholders(compiler, mainElement);
+    FunctionExpression mainNode = mainElement.parseNode(compiler);
+    Block body = mainNode.body;
+    FunctionDeclaration functionDeclaration = body.statements.nodes.head;
+    FunctionExpression fooNode = functionDeclaration.function;
+    LocalPlaceholder fooPlaceholder =
+        collector.functionScopes[mainElement].localPlaceholders.first;
+    Expect.isTrue(fooPlaceholder.nodes.contains(fooNode.name));
+  }));
 }
 
 testTypeVariablesAreRenamed() {
   // Somewhat a hack: we require all the references of the identifier
   // to be renamed in the same way for the whole library. Hence
   // if we have a class and type variable with the same name, they
   // both should be renamed.
   var librarySrc = '''
 library mylib;
 typedef void MyFunction<T extends num>(T arg);
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   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor();
 }