fix #551, add toJSON to ZoneTask to prevent cyclic error

lib/zone.ts

@@ -962,6 +962,22 @@ const Zone: ZoneType = (function(global: any) {
         return Object.prototype.toString.call(this);
       }
     }
+
+    // add toJSON method to prevent cyclic error when
+    // call JSON.stringify(zoneTask)
+    public toJSON() {
+      return {
+        type: this.type,
+        source: this.source,
+        data: this.data,
+        zone: this.zone.name,
+        invoke: this.invoke,
+        scheduleFn: this.scheduleFn,
+        cancelFn: this.cancelFn,
+        runCount: this.runCount,
+        callback: this.callback
+      };
+    }
   }
 
   interface UncaughtPromiseError extends Error {

test/common/zone.spec.ts

@@ -231,6 +231,22 @@ describe('Zone', function() {
 
       macro.invoke();
     });
+
+    it('should convert task to json without cyclic error', () => {
+      const z = Zone.current;
+      const event = z.scheduleEventTask('test', () => {}, null, noop, noop);
+      const micro = z.scheduleMicroTask('test', () => {});
+      const macro = z.scheduleMacroTask('test', () => {}, null, noop, noop);
+      expect(function() {
+        JSON.stringify(event);
+      }).not.toThrow();
+      expect(function() {
+        JSON.stringify(micro);
+      }).not.toThrow();
+      expect(function() {
+        JSON.stringify(macro);
+      }).not.toThrow();
+    });
   });
 });