Add symbol to identify a ReactTestMount instance.

src/renderers/testing/ReactTestRenderer.js

@@ -74,11 +74,15 @@ ReactTestComponent.prototype.toJSON = function() {
       childrenJSON.push(json);
     }
   }
-  return {
+  var object = {
     type: this._currentElement.type,
     props: props,
     children: childrenJSON.length ? childrenJSON : null,
   };
+  Object.defineProperty(object, '$$typeof', {
+    value: Symbol.for('react.test.json'),
+  });
+  return object;
 };
 Object.assign(ReactTestComponent.prototype, ReactMultiChild.Mixin);
 

src/renderers/testing/__tests__/ReactTestRenderer-test.js

@@ -28,6 +28,22 @@ describe('ReactTestRenderer', function() {
     });
   });
 
+  it('exposes a type flag', function() {
+    function Link() {
+      return <a role="link" />;
+    }
+    var renderer = ReactTestRenderer.create(<Link />);
+    var object = renderer.toJSON();
+    expect(object.$$typeof).toBe(Symbol.for('react.test.json'));
+
+    // $$typeof should not be enumerable.
+    for (var key in object) {
+      if (Object.prototype.hasOwnProperty.call(object, key)) {
+        expect(key).not.toBe('$$typeof');
+      }
+    }
+  });
+
   it('renders some basics with an update', function() {
     var renders = 0;
     var Component = React.createClass({