First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/rewrite_better_user_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.
 // Test that we get rid of duplicate type guards on a field when that
 // field is being gvn'ed.
 
 import 'compiler_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 const String TEST = r"""
 class A {
   var field = 52;
   foo() {
     var a = this.field;
     while (a + 42 == 42);
     // This field get should be GVN'ed
     a = this.field;
     while (a + 87 == 87);
     field = 'bar';
   }
 }
 
 main() {
   while (true) new A().foo();
 }
 """;
 
 main() {
-  String generated = compileAll(TEST);
-  RegExp regexp = new RegExp('foo\\\$0\\\$bailout');
-  Iterator matches = regexp.allMatches(generated).iterator;
+  asyncTest(() => compileAll(TEST).then((generated) {
+    RegExp regexp = new RegExp('foo\\\$0\\\$bailout');
+    Iterator matches = regexp.allMatches(generated).iterator;
 
-  // We check that there is only one call to the bailout method.
-  // One match for the call, one for the definition.
-  checkNumberOfMatches(matches, 2);
+    // We check that there is only one call to the bailout method.
+    // One match for the call, one for the definition.
+    checkNumberOfMatches(matches, 2);
+  }));
 }