anchor.diff:      Mock primaryInput;
       SpriteAssetProviderMock provider;
   
  -    void buildTransformer(String html, List<Asset> assets, [SpriteSheet spriteSheet]) {
  +    void buildTransformer(String html, {List<Asset> assets: const [], SpriteSheet spriteSheet}) {
         primaryInput.when(callsTo("readAsString")).alwaysReturn(new Future.value(html));
         var spriteSheetEngine = new TestSpriteSheetEngine()
             ..when(callsTo("generate")).alwaysReturn(new Future.value(spriteSheet));
  
positive.code:     });
  
      describe("when provider returns empty list of assets", () {
        beforeEach(() => buildTransformer(htmlWithCss, []));
  
        it("no outputs are added to transform", () {
          transformer.apply(transform).then(expectAsync((_) {
            transform.getLogs(callsTo("addOutput")).verify(neverHappened);
          }));
  
negative.code:     });
  
      describe("when primary input has linked stylesheets", () {
        beforeEach(() => buildTransformer(htmlWithCss));
  
        it("requests provider for assets", () {
          transformer.apply(transform).then(expectAsync((_) => provider
  