feat(Scope): async auto-flush $evalAsync queue when outside of $digest

This change causes a new $digest to be scheduled in the next tick if
a task was was sent to the $evalAsync queue from outside of a $digest
or an $apply.

While this mode of operation is not common for most of the user code,
this change means that $q promises that utilze $evalAsync queue to
guarantee asynchronicity of promise apis will now also resolve outside
of a $digest, which turned out to be a big pain point for some developers.

The implementation ensures that we don't do more work than needed and
that we coalese as much work as possible into a single $digest.

The use of $browser instead of setTimeout ensures that we can mock out
and control the scheduling of "auto-flush", which should in theory
allow all of the existing code and tests to work without negative
side-effects.

Closes #3539
Closes #2438

src/ng/rootScope.js

@@ -69,8 +69,8 @@ function $RootScopeProvider(){
     return TTL;
   };
 
-  this.$get = ['$injector', '$exceptionHandler', '$parse',
-      function( $injector,   $exceptionHandler,   $parse) {
+  this.$get = ['$injector', '$exceptionHandler', '$parse', '$browser',
+      function( $injector,   $exceptionHandler,   $parse,   $browser) {
 
     /**
      * @ngdoc function
@@ -666,20 +666,33 @@ function $RootScopeProvider(){
        *
        * The `$evalAsync` makes no guarantees as to when the `expression` will be executed, only that:
        *
-       *   - it will execute in the current script execution context (before any DOM rendering).
-       *   - at least one {@link ng.$rootScope.Scope#$digest $digest cycle} will be performed after
-       *     `expression` execution.
+       *   - it will execute after the function that schedule the evaluation is done running (preferably before DOM rendering).
+       *   - at least one {@link ng.$rootScope.Scope#$digest $digest cycle} will be performed after `expression` execution.
        *
        * Any exceptions from the execution of the expression are forwarded to the
        * {@link ng.$exceptionHandler $exceptionHandler} service.
        *
+       * __Note:__ if this function is called outside of `$digest` cycle, a new $digest cycle will be scheduled.
+       * It is however encouraged to always call code that changes the model from withing an `$apply` call.
+       * That includes code evaluated via `$evalAsync`.
+       *
        * @param {(string|function())=} expression An angular expression to be executed.
        *
        *    - `string`: execute using the rules as defined in  {@link guide/expression expression}.
        *    - `function(scope)`: execute the function with the current `scope` parameter.
        *
        */
       $evalAsync: function(expr) {
+        // if we are outside of an $digest loop and this is the first time we are scheduling async task also schedule
+        // async auto-flush
+        if (!$rootScope.$$phase && !$rootScope.$$asyncQueue.length) {
+          $browser.defer(function() {
+            if ($rootScope.$$asyncQueue.length) {
+              $rootScope.$digest();
+            }
+          });
+        }
+
         this.$$asyncQueue.push(expr);
       },
 

src/ng/timeout.js

@@ -36,7 +36,7 @@ function $TimeoutProvider() {
       var deferred = $q.defer(),
           promise = deferred.promise,
           skipApply = (isDefined(invokeApply) && !invokeApply),
-          timeoutId, cleanup;
+          timeoutId;
 
       timeoutId = $browser.defer(function() {
         try {

test/ng/rootScopeSpec.js

@@ -705,6 +705,57 @@ describe('Scope', function() {
       expect(isolateScope.$$asyncQueue).toBe($rootScope.$$asyncQueue);
       expect($rootScope.$$asyncQueue).toEqual(['rootExpression', 'childExpression', 'isolateExpression']);
     }));
+
+
+    describe('auto-flushing when queueing outside of an $apply', function() {
+      var log, $rootScope, $browser;
+
+      beforeEach(inject(function(_log_, _$rootScope_, _$browser_) {
+        log = _log_;
+        $rootScope = _$rootScope_;
+        $browser = _$browser_;
+      }));
+
+
+      it('should auto-flush the queue asynchronously and trigger digest', function() {
+        $rootScope.$evalAsync(log.fn('eval-ed!'));
+        $rootScope.$watch(log.fn('digesting'));
+        expect(log).toEqual([]);
+
+        $browser.defer.flush(0);
+
+        expect(log).toEqual(['eval-ed!', 'digesting', 'digesting']);
+      });
+
+
+      it('should not trigger digest asynchronously if the queue is empty in the next tick', function() {
+        $rootScope.$evalAsync(log.fn('eval-ed!'));
+        $rootScope.$watch(log.fn('digesting'));
+        expect(log).toEqual([]);
+
+        $rootScope.$digest();
+
+        expect(log).toEqual(['eval-ed!', 'digesting', 'digesting']);
+        log.reset();
+
+        $browser.defer.flush(0);
+
+        expect(log).toEqual([]);
+      });
+
+
+      it('should not schedule more than one auto-flush task', function() {
+        $rootScope.$evalAsync(log.fn('eval-ed 1!'));
+        $rootScope.$evalAsync(log.fn('eval-ed 2!'));
+
+        $browser.defer.flush(0);
+        expect(log).toEqual(['eval-ed 1!', 'eval-ed 2!']);
+
+        expect(function() {
+          $browser.defer.flush(0);
+        }).toThrow('No deferred tasks with delay up to 0ms to be flushed!');
+      });
+    });
   });