First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/list_tracer_length_test.dart

 // Copyright (c) 2013, 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 'compiler_helper.dart';
 
 const String TEST1 = r"""
 var a = [42];
 main() {
@@ skipping 45 matching lines @@
 doIt(a) {
   a.clear();
   foo(a);
 }
 main() {
   foo(null);
 }
 """;
 
 void checkRangeError(String test, {bool hasRangeError}) {
-  String generated = compileAll(test);
-  Expect.equals(hasRangeError, generated.contains('ioore'));
+  compileAll(test).then((generated) {
+    Expect.equals(hasRangeError, generated.contains('ioore'));
+  });
 }
 
 main() {
   checkRangeError(TEST1, hasRangeError: false);
   checkRangeError(TEST2('insert', 'null, null'), hasRangeError: true);
   checkRangeError(TEST2('add', 'null'), hasRangeError: true);
   checkRangeError(TEST2('clear', ''), hasRangeError: true);
   checkRangeError(TEST2('toString', ''), hasRangeError: false);
   checkRangeError(TEST3, hasRangeError: false);
   checkRangeError(TEST4, hasRangeError: true);
   checkRangeError(TEST5, hasRangeError: true);
   checkRangeError(TEST6, hasRangeError: true);
 }