feat(change_detection.js): export null pipes

rather then having developers creating their own null pipes Closes angular#1624

Author: PatrickJS

modules/angular2/change_detection.ts

@@ -47,6 +47,7 @@ export {ChangeDetectorRef} from './src/change_detection/change_detector_ref';
 export {PipeRegistry} from './src/change_detection/pipes/pipe_registry';
 export {uninitialized} from './src/change_detection/change_detection_util';
 export {WrappedValue, Pipe} from './src/change_detection/pipes/pipe';
+export {NullPipe, NullPipeFactory} from './src/change_detection/pipes/null_pipe';
 export {
  defaultPipes,
  DynamicChangeDetection,