First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/tree_shaking_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 'compiler_helper.dart';
 
 const String TEST = r"""
 class A {
   foo() => bar();
   bar() => 42;
 }
 class B extends A {
   bar() => 54;
 }
 class C implements A {
   bar() => 68;
 }
 main() {
   new A();
   new B();
   new C();
   new A().foo();
 }
 """;
 
 void main() {
-  String generated = compileAll(TEST);
-  Expect.isTrue(generated.contains('return 42'));
-  Expect.isTrue(generated.contains('return 54'));
-  Expect.isFalse(generated.contains('return 68'));
+  compileAll(TEST).then((generated) {
+    Expect.isTrue(generated.contains('return 42'));
+    Expect.isTrue(generated.contains('return 54'));
+    Expect.isFalse(generated.contains('return 68'));
+  });
 }