First pass at asynchronous input loading in dart2js.

tests/utils/dummy_compiler_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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
+import "package:async_helper/async_helper.dart";
 
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 
 Future<String> provider(Uri uri) {
   String source;
   if (uri.scheme == "main") {
     source = "main() {}";
   } else if (uri.scheme == "lib") {
     if (uri.path.endsWith("/core.dart")) {
       source = """
@@ skipping 74 matching lines @@
 
 void handler(Uri uri, int begin, int end, String message, Diagnostic kind) {
   if (uri == null) {
     print('$kind: $message');
   } else {
     print('$uri:$begin:$end: $kind: $message');
   }
 }
 
 main() {
+  asyncStart();
   Future<String> result =
       compile(new Uri(scheme: 'main'),
               new Uri(scheme: 'lib', path: '/'),
               new Uri(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
-  });
+  }).whenComplete(() => asyncEnd());
 }