Make Completers asynchronous.

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.
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
 import 'dart:uri';
 
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 
 Future<String> provider(Uri uri) {
-  Completer<String> completer = new Completer<String>();
   String source;
   if (uri.scheme == "main") {
     source = "main() {}";
   } else if (uri.scheme == "lib") {
     if (uri.path.endsWith("/core.dart")) {
       source = """library core;
                   class Object {
                     operator==(other) {}
                   }
                   class Type {}
@@ skipping 48 matching lines @@
       source = 'library jshelper; class JSInvocationMirror {} '
                'class ConstantMap {} class TypeImpl {}';
     } else if (uri.path.endsWith('isolate_helper.dart')) {
       source = 'library isolatehelper; class _WorkerStub {}';
     } else {
       source = "library lib;";
     }
   } else {
    throw "unexpected URI $uri";
   }
-  completer.complete(source);
-  return completer.future;
+  return new Future.value(source);
 }
 
 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() {
   Future<String> result =
       compile(new Uri.fromComponents(scheme: 'main'),
               new Uri.fromComponents(scheme: 'lib', path: '/'),
               new Uri.fromComponents(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   });
 }