Access to imports in the VM's runtime mirrors. Extend test coverage of the source mirrors.

sdk/lib/_internal/lib/js_mirrors.dart

Index: sdk/lib/_internal/lib/js_mirrors.dart
diff --git a/sdk/lib/_internal/lib/js_mirrors.dart b/sdk/lib/_internal/lib/js_mirrors.dart
index 24e784023e4da9998711c40b2ffd8b4ccdce986e..46f7a26e15e7f3b478b2c16b36608a7b3c38136f 100644
--- a/sdk/lib/_internal/lib/js_mirrors.dart
+++ b/sdk/lib/_internal/lib/js_mirrors.dart
@@ -488,6 +488,9 @@ class JsLibraryMirror extends JsDeclarationMirror with JsObjectMirror
   // TODO(ahe): Implement this.
   Function operator [](Symbol name)
       => throw new UnimplementedError();
+
+  List<LibraryDependencyMirror> get libraryDependencies
+      => throw new UnimplementedError();
 }
 
 String n(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);