Add implementation of Isolate.resolvePackageUri for dart2js.

sdk/lib/_internal/js_runtime/lib/isolate_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
index c181c6da962f281f1358c6798ca3cd856f2a8c92..18da81df1d6d1ad555502b35ec69f4bb68442f7b 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
@@ -751,6 +751,14 @@ class IsolateNatives {
 
   static String thisScript = computeThisScript();
 
+  /// Returns the base path added to Uri.base to resolve `package:` Uris.
+  ///
+  /// This is used by `Isolate.resolvePackageUri` to load resources. The default
+  /// value is `packages/` but users can override this by using the
+  /// `defaultPackagesBase` hook.
+  static String get packagesBase =>
+      JS('String', r'self.defaultPackagesBase || "packages/"');
+
   /// Associates an ID with a native worker object.
   static final Expando<int> workerIds = new Expando<int>();