nextTick now keeps reference to correct scope.

Previously, `nextTick` would decide whether to run the functions based
on the first `scope` argument passed during a given digest cycle.  This
could fail in multiple scenarios ...

1) The first `nextTick` call didn't supply a scope, but subsequent calls did.
2) Multiple components (with different scopes) registered callbacks during the same digest cycle.

This could lead to callbacks being executed when they shouldn't be (if
the first scope registered wasn't $$destroyed, this _this_ scope was),
or callbacks not being executed when they should be (the first scope was
$$destroyed, but this scope wasn't).

I believe that this fixes #8358

src/core/util/util.js

@@ -591,7 +591,7 @@ function UtilFactory($document, $timeout, $compile, $rootScope, $$mdAnimate, $in
       var queue = nextTick.queue || [];
 
       //-- add callback to the queue
-      queue.push(callback);
+      queue.push({scope: scope, callback: callback});
 
       //-- set default value for digest
       if (digest == null) digest = true;
@@ -610,16 +610,18 @@ function UtilFactory($document, $timeout, $compile, $rootScope, $$mdAnimate, $in
        * Trigger digest if necessary
        */
       function processQueue() {
-        var skip = scope && scope.$$destroyed;
-        var queue = !skip ? nextTick.queue : [];
-        var digest = !skip ? nextTick.digest : null;
+        var queue = nextTick.queue;
+        var digest = nextTick.digest;
 
         nextTick.queue = [];
         nextTick.timeout = null;
         nextTick.digest = false;
 
-        queue.forEach(function(callback) {
-          callback();
+        queue.forEach(function(queueItem) {
+          var skip = queueItem.scope && queueItem.scope.$$destroyed;
+          if (!skip) {
+            queueItem.callback();
+          }
         });
 
         if (digest) $rootScope.$digest();