First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/library_load_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 'mock_compiler.dart';
-
-class ScanMockCompiler extends MockCompiler {
-  ScanMockCompiler() {
-    isolateHelperLibrary = null;
-    foreignLibrary = null;
-  }
-
-  LibraryElement scanBuiltinLibrary(String filename) {
-    return createLibrary(filename, "main(){}");
-  }
-}
-
-void main() {
-  Compiler compiler = new ScanMockCompiler();
-  Expect.equals(null, compiler.isolateHelperLibrary);
-  Expect.equals(null, compiler.foreignLibrary);
-  compiler.onLibraryLoaded(mockLibrary(compiler, "mock"),
-                            new Uri(scheme: 'dart', path: '_isolate_helper'));
-  Expect.isTrue(compiler.isolateHelperLibrary != null);
-  compiler.onLibraryLoaded(mockLibrary(compiler, "mock"),
-                            new Uri(scheme: 'dart', path: '_foreign_helper'));
-  Expect.isTrue(compiler.isolateHelperLibrary != null);
-  Expect.equals(new Uri(scheme: 'dart', path: '_isolate_helper'),
-                 compiler.isolateHelperLibrary.canonicalUri);
-  Expect.isTrue(compiler.foreignLibrary != null);
-  Expect.equals(new Uri(scheme: 'dart', path: '_foreign_helper'),
-                 compiler.foreignLibrary.canonicalUri);
-}