fix(codegen): event handler has boolean return type

modules/angular2/src/compiler/view_compiler/event_binder.ts

@@ -87,7 +87,7 @@ export class CompileEventListener {
            [
              new o.ReturnStatement(
                  o.THIS_EXPR.callMethod(this._methodName, [EventHandlerVars.event]))
-           ])
+           ], o.BOOL_TYPE)
     ]);
     if (isPresent(this.eventTarget)) {
       listenExpr = ViewProperties.renderer.callMethod(
@@ -162,4 +162,4 @@ function convertStmtIntoExpression(stmt: o.Statement): o.Expression {
 
 function santitizeEventName(name: string): string {
   return StringWrapper.replaceAll(name, /[^a-zA-Z_]/g, '_');
-}
+}