fix(render): only look for content tags in views that might have them.

Largetable benchmark with `interpolationAttr` and 200 rows / 20 columns:
Time for destroy/create pair dropped from about 1260ms to about 150ms.

Related to #2298, but does not really fix it as we are still slow
if people are using `<content>`.

Closes #2297

modules/angular2/src/render/dom/shadow_dom/light_dom.ts

@@ -52,6 +52,13 @@ export class LightDom {
 
   // Collects the Content directives from the view and all its child views
   private _collectAllContentTags(view: viewModule.DomView, acc: List<Content>): List<Content> {
+    // Note: exiting early here is important as we call this function for every view
+    // that is added, so we have O(n^2) runtime.
+    // TODO(tbosch): fix the root problem, see
+    // https://github.com/angular/angular/issues/2298
+    if (view.proto.transitiveContentTagCount === 0) {
+      return acc;
+    }
     var contentTags = view.contentTags;
     var vcs = view.viewContainers;
     for (var i = 0; i < vcs.length; i++) {

modules/angular2/src/render/dom/view/proto_view.ts

@@ -25,10 +25,13 @@ export class DomProtoView {
   elementBinders: List<ElementBinder>;
   isTemplateElement: boolean;
   rootBindingOffset: number;
+  // the number of content tags seen in this or any child proto view.
+  transitiveContentTagCount: number;
 
-  constructor({elementBinders, element}) {
+  constructor({elementBinders, element, transitiveContentTagCount}) {
     this.element = element;
     this.elementBinders = elementBinders;
+    this.transitiveContentTagCount = transitiveContentTagCount;
     this.isTemplateElement = DOM.isTemplateElement(this.element);
     this.rootBindingOffset =
         (isPresent(this.element) && DOM.hasClass(this.element, NG_BINDING_CLASS)) ? 1 : 0;

modules/angular2/src/render/dom/view/proto_view_builder.ts

@@ -54,6 +54,7 @@ export class ProtoViewBuilder {
     var renderElementBinders = [];
 
     var apiElementBinders = [];
+    var transitiveContentTagCount = 0;
     ListWrapper.forEach(this.elements, (ebb) => {
       var propertySetters = MapWrapper.create();
       var hostActions = MapWrapper.create();
@@ -82,6 +83,14 @@ export class ProtoViewBuilder {
       });
 
       var nestedProtoView = isPresent(ebb.nestedProtoView) ? ebb.nestedProtoView.build() : null;
+      var nestedRenderProtoView =
+          isPresent(nestedProtoView) ? resolveInternalDomProtoView(nestedProtoView.render) : null;
+      if (isPresent(nestedRenderProtoView)) {
+        transitiveContentTagCount += nestedRenderProtoView.transitiveContentTagCount;
+      }
+      if (isPresent(ebb.contentTagSelector)) {
+        transitiveContentTagCount++;
+      }
       var parentIndex = isPresent(ebb.parent) ? ebb.parent.index : -1;
       ListWrapper.push(apiElementBinders, new api.ElementBinder({
         index: ebb.index,
@@ -112,8 +121,11 @@ export class ProtoViewBuilder {
                        }));
     });
     return new api.ProtoViewDto({
-      render: new DomProtoViewRef(
-          new DomProtoView({element: this.rootElement, elementBinders: renderElementBinders})),
+      render: new DomProtoViewRef(new DomProtoView({
+        element: this.rootElement,
+        elementBinders: renderElementBinders,
+        transitiveContentTagCount: transitiveContentTagCount
+      })),
       type: this.type,
       elementBinders: apiElementBinders,
       variableBindings: this.variableBindings