First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/mirror_tree_shaking_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.
 
 // Test that tree-shaking hasn't been turned off.
 
 import 'package:expect/expect.dart';
 import 'memory_source_file_helper.dart';
 
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart'
@@ skipping 22 matching lines @@
   EventSink<String> outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 
   Compiler compiler = new Compiler(provider.readStringFromUri,
                                    outputProvider,
                                    diagnosticHandler,
                                    libraryRoot,
                                    packageRoot,
                                    []);
-  compiler.run(Uri.parse('memory:main.dart'));
-  Expect.isFalse(compiler.compilationFailed);
-  Expect.isFalse(compiler.enqueuer.resolution.hasEnqueuedEverything);
-  Expect.isFalse(compiler.enqueuer.codegen.hasEnqueuedEverything);
-  Expect.isFalse(compiler.disableTypeInference);
+  compiler.run(Uri.parse('memory:main.dart')).then((_) {
+    Expect.isFalse(compiler.compilationFailed);
+    Expect.isFalse(compiler.enqueuer.resolution.hasEnqueuedEverything);
+    Expect.isFalse(compiler.enqueuer.codegen.hasEnqueuedEverything);
+    Expect.isFalse(compiler.disableTypeInference);
+  });
 }
 
 const Map MEMORY_SOURCE_FILES = const {
   'main.dart': r"""
 import 'dart:mirrors';
 
 class Foo {
   noSuchMethod(invocation) {
     print('Invoked ${MirrorSystem.getName(invocation.memberName)}');
     return reflect('foobar').delegate(invocation);
   }
 }
 
 void main() {
   print(new Foo().substring(3));
 }
 """,
 };