feat(Zone): add unique id to each zone

Closes #45

Author: btford

Diff for: test/zone.spec.js

@@ -6,6 +6,14 @@ describe('Zone', function () {
     zone.mark = 'root';
   });
 
+  it('should have an id', function () {
+    expect(zone.$id).toBeDefined();
+  });
+
+  it('forked zones should have a greater id than their parent', function () {
+    expect(zone.fork().$id).toBeGreaterThan(zone.$id);
+  });
+
   describe('hooks', function () {
 
     it('should fire beforeTask before a zone runs a function', function () {

Diff for: zone.js

@@ -45,6 +45,8 @@ function Zone(parentZone, data) {
     }
   });
 
+  zone.$id = ++Zone.nextId;
+
   return zone;
 }
 
@@ -166,6 +168,8 @@ Zone.patchSetClearFn = function (obj, fnNames) {
   });
 };
 
+Zone.nextId = 1;
+
 
 Zone.patchSetFn = function (obj, fnNames) {
   fnNames.forEach(function (name) {