fix: don't fork new zones for callbacks from the root zone

It does not make sense for us to create new zones for callbacks from the root zone since we care
only about callbacks from installed custom zones. This reduces the overhead of zones.

Closes angular#92

test/zone.spec.js

@@ -149,6 +149,30 @@ describe('Zone', function () {
   });
 
 
+  describe('bind', function() {
+
+    it('should execute all callbacks from root zone without forking zones', function(done) {
+      // using setTimeout for the test which relies on patching via bind
+      setTimeout(function() {
+        expect(zone.isRootZone()).toBe(true);
+        done();
+      });
+    });
+
+
+    it('should fork a zone for non-root zone', function(done) {
+      // using setTimeout for the test which relies on patching via bind
+      var childZone = zone.fork();
+      childZone.run(function() {
+        setTimeout(function() {
+          expect(zone.parent).toBe(childZone);
+          done();
+        });
+      });
+    });
+  });
+
+
   describe('fork', function () {
     it('should fork deep copy', function () {
       var protoZone = { too: { deep: true } },

zone.js

@@ -65,7 +65,7 @@ Zone.prototype = {
 
   bind: function (fn, skipEnqueue) {
     skipEnqueue || this.enqueueTask(fn);
-    var zone = this.fork();
+    var zone = this.isRootZone() ? this : this.fork();
     return function zoneBoundFn() {
       return zone.run(fn, this, arguments);
     };