First pass at asynchronous input loading in dart2js.

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 'parser_helper.dart';
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart' as leg;
 import '../../../sdk/lib/_internal/compiler/implementation/dart_backend/dart_backend.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/tree/tree.dart';
 
 const coreLib = r'''
 library corelib;
@@ skipping 105 matching lines @@
       Expect.fail('$uri: $begin-$end: $message [$kind]');
     }
   }
 
   final options = <String>['--output-type=dart'];
   // Some tests below are using dart:io.
   options.add('--categories=Client,Server');
   if (minify) options.add('--minify');
   if (stripTypes) options.add('--force-strip=types');
 
-  compile(
+  asyncTest(() => compile(
       scriptUri,
       fileUri('libraryRoot/'),
       fileUri('packageRoot/'),
       provider,
       handler,
-      options).then(continuation);
+      options).then(continuation));
 }
 
 testSimpleFileUnparse() {
   final src = '''
 should_be_dropped() {
 }
 
 should_be_kept() {
 }
 
@@ skipping 577 matching lines @@
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor();
 }