First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/mirrors_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 "package:async_helper/async_helper.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors_util.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/filenames.dart'
        show currentDirectory, nativeToUriPath;
 import '../../../sdk/lib/_internal/compiler/implementation/source_file_provider.dart';
 
 import 'dart:io';
 
 final Uri DART_MIRRORS_URI = new Uri(scheme: 'dart', path: 'mirrors');
@@ skipping 25 matching lines @@
 }
 
 main() {
   Uri scriptUri =
       currentDirectory.resolve(nativeToUriPath(Platform.script));
   Uri libUri = scriptUri.resolve('../../../sdk/');
   Uri inputUri = scriptUri.resolve('mirrors_helper.dart');
   var provider = new SourceFileProvider();
   var diagnosticHandler =
         new FormattingDiagnosticHandler(provider).diagnosticHandler;
+  asyncStart();
   var result = analyze([inputUri], libUri, null,
                        provider.readStringFromUri, diagnosticHandler,
                        <String>['--preserve-comments']);
   result.then((MirrorSystem mirrors) {
     test(mirrors);
-  });
+  }).whenComplete(() => asyncEnd());
 }
 
 void test(MirrorSystem mirrors) {
   Expect.isNotNull(mirrors, "No mirror system returned from compilation");
 
   var libraries = mirrors.libraries;
   Expect.isNotNull(libraries, "No libraries map returned");
   Expect.isFalse(libraries.isEmpty, "Empty libraries map returned");
 
   var helperLibrary = findMirror(libraries.values, "mirrors_helper");
@@ skipping 957 matching lines @@
   Expect.isTrue(privateFactoryConstructor.isPrivate);
   Expect.isFalse(privateFactoryConstructor.isConstConstructor);
   Expect.isFalse(privateFactoryConstructor.isRedirectingConstructor);
   Expect.isFalse(privateFactoryConstructor.isGenerativeConstructor);
   Expect.isTrue(privateFactoryConstructor.isFactoryConstructor);
 
   var metadata = privateClass.metadata;
   Expect.isNotNull(metadata);
   Expect.equals(0, metadata.length);
 }