perf($q): small $q performance optimization

Only generate a new promise when `then` receives some non-undefined parameter

Closes angular#12535

Author: lgalfaso

lib/promises-aplus/promises-aplus-test-adapter.js

@@ -4,6 +4,8 @@
 var isFunction = function isFunction(value){return typeof value == 'function';};
 var isPromiseLike = function isPromiseLike(obj) {return obj && isFunction(obj.then);};
 var isObject = function isObject(value){return value != null && typeof value === 'object';};
+var isUndefined = function isUndefined(value) {return typeof value === 'undefined';};
+
 var minErr = function minErr (module, constructor) {
   return function (){
     var ErrorConstructor = constructor || Error;

src/ng/q.js

@@ -274,6 +274,9 @@ function qFactory(nextTick, exceptionHandler) {
 
   extend(Promise.prototype, {
     then: function(onFulfilled, onRejected, progressBack) {
+      if (isUndefined(onFulfilled) && isUndefined(onRejected) && isUndefined(progressBack)) {
+        return this;
+      }
       var result = new Deferred();
 
       this.$$state.pending = this.$$state.pending || [];