Combining style tags into a single style tag helps performance

[irfantusneem]

Combining style tags into a single style tag helps performance. The code below is a change to shadow_dom_component_factory.dart to cache the combined style tags for a component so only a single tag is output.

Ran the code against 10,000 instances of a button list component 5 times and it showed a 3% performance improvement.

The code below is a bit of a hack since it assumes there are only 2 styles but it can be cleaned up to handle more.

            // ############NEW CODE DOWN##################
  static Map _twoListMap = {};
            // ############NEW CODE UP##################
  Function call(dom.Element element) {
    return (DirectiveInjector injector, Scope scope, NgBaseCss baseCss,
            EventHandler _) {
      var s = traceEnter(View_createComponent);
      try {
        var shadowDom = element.createShadowRoot()
          ..applyAuthorStyles = _component.applyAuthorStyles
          ..resetStyleInheritance = _component.resetStyleInheritance;

        var shadowScope = scope.createChild(new HashMap()); // Isolate

        async.Future<Iterable<dom.StyleElement>> cssFuture;
        if (_component.useNgBaseCss == true) {
          cssFuture = async.Future.wait([async.Future.wait(baseCss.urls.map(_styleFuture)), _styleElementsFuture]).then((twoLists) {
            assert(twoLists.length == 2);
            var cssList = []..addAll(twoLists[0])
            ..addAll(twoLists[1]);
            // ############NEW CODE DOWN##################
            var css1 = cssList[0];
            var css2 = cssList[1];
            var styleMap = _twoListMap[css1];
            if (styleMap == null) {
              styleMap = new Map();
              _twoListMap[css1] = styleMap;
            }
            var styleDom = styleMap[css2];
            if (styleDom == null) {
              styleDom = new dom.StyleElement()..appendText(css1.text + css2.text);
              styleDom = [ styleDom ];
              styleMap[css2] = styleDom;
            }
            return styleDom;
            // ############NEW CODE  UP##################
            // return cssList;
          });
        } else {
          cssFuture = _styleElementsFuture;
        }

[mhevery]

will be addressed in the new compiler.