Add implementation of Isolate.resolvePackageUri for dart2js.

pkg/compiler/lib/src/js_emitter/headers.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.js_emitter.headers;
 
 import '../compiler.dart' show Compiler;
 
 String generatedBy(Compiler compiler, {String flavor: ""}) {
   String suffix = '';
@@ skipping 14 matching lines @@
 // dartMainRunner(main, args):
 //    if this function is defined, the Dart [main] method will not be invoked
 //    directly. Instead, a closure that will invoke [main], and its arguments
 //    [args] is passed to [dartMainRunner].
 //
 // dartDeferredLibraryLoader(uri, successCallback, errorCallback):
 //    if this function is defined, it will be called when a deferred library
 //    is loaded. It should load and eval the javascript of `uri`, and call
 //    successCallback. If it fails to do so, it should call errorCallback with
 //    an error.
+//
+// defaultPackagesBase:
+//    Override the location where `package:` uris are resolved from. By default
+//    they are resolved under "packages/" from the current window location.
 """;