Add implementation of Isolate.resolvePackageUri for dart2js.

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

Index: pkg/compiler/lib/src/js_emitter/headers.dart
diff --git a/pkg/compiler/lib/src/js_emitter/headers.dart b/pkg/compiler/lib/src/js_emitter/headers.dart
index 905cb04f048f4aae1256af388d55e2dd22abd492..ad9e783403d0b6f55c1bc6a676b848ac84f095ea 100644
--- a/pkg/compiler/lib/src/js_emitter/headers.dart
+++ b/pkg/compiler/lib/src/js_emitter/headers.dart
@@ -32,4 +32,8 @@ const String HOOKS_API_USAGE = """
 //    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.
 """;