feat(events): support preventdefault

Fixes #1039
Closes #1397

modules/angular2/src/core/annotations/annotations.js

@@ -391,6 +391,7 @@ export class Directive extends Injectable {
    *
    * - `event1`: the DOM event that the directive listens to.
    * - `statement`: the statement to execute when the event occurs.
+   * If the evalutation of the statement returns `false`, then `preventDefault`is applied on the DOM event.
    *
    * To listen to global events, a target must be added to the event name.
    * The target can be `window`, `document` or `body`.

modules/angular2/src/core/compiler/view.js

@@ -131,17 +131,17 @@ export class AppView {
   }
 
   // implementation of EventDispatcher#dispatchEvent
-  dispatchEvent(
-    elementIndex:number, eventName:string, locals:Map<string, any>
-  ):void {
+  // returns false if preventDefault must be applied to the DOM event
+  dispatchEvent(elementIndex:number, eventName:string, locals:Map<string, any>): boolean {
     // Most of the time the event will be fired only when the view is in the live document.
     // However, in a rare circumstance the view might get dehydrated, in between the event
     // queuing up and firing.
+    var allowDefaultBehavior = true;
     if (this.hydrated()) {
       var elBinder = this.proto.elementBinders[elementIndex];
-      if (isBlank(elBinder.hostListeners)) return;
+      if (isBlank(elBinder.hostListeners)) return allowDefaultBehavior;
       var eventMap = elBinder.hostListeners[eventName];
-      if (isBlank(eventMap)) return;
+      if (isBlank(eventMap)) return allowDefaultBehavior;
       MapWrapper.forEach(eventMap, (expr, directiveIndex) => {
         var context;
         if (directiveIndex === -1) {
@@ -149,9 +149,13 @@ export class AppView {
         } else {
           context = this.elementInjectors[elementIndex].getDirectiveAtIndex(directiveIndex);
         }
-        expr.eval(context, new Locals(this.locals, locals));
+        var result = expr.eval(context, new Locals(this.locals, locals));
+        if (isPresent(result)) {
+          allowDefaultBehavior = allowDefaultBehavior && result;  
+        }
       });
     }
+    return allowDefaultBehavior;
   }
 }
 

modules/angular2/src/render/dom/view/view.js

@@ -84,7 +84,8 @@ export class RenderView {
     this._eventDispatcher = dispatcher;
   }
 
-  dispatchEvent(elementIndex, eventName, event) {
+  dispatchEvent(elementIndex, eventName, event): boolean {
+    var allowDefaultBehavior = true;
     if (isPresent(this._eventDispatcher)) {
       var evalLocals = MapWrapper.create();
       MapWrapper.set(evalLocals, '$event', event);
@@ -92,7 +93,11 @@ export class RenderView {
       // out of action expressions
       // var localValues = this.proto.elementBinders[elementIndex].eventLocals.eval(null, new Locals(null, evalLocals));
       // this._eventDispatcher.dispatchEvent(elementIndex, eventName, localValues);
-      this._eventDispatcher.dispatchEvent(elementIndex, eventName, evalLocals);
+      allowDefaultBehavior = this._eventDispatcher.dispatchEvent(elementIndex, eventName, evalLocals);
+      if (!allowDefaultBehavior) {
+        event.preventDefault();
+      }
     }
+    return allowDefaultBehavior;
   }
 }