chore(changelog): mention breaking test API change

Author: yjbanov

CHANGELOG.md

@@ -101,7 +101,10 @@
 * **test_lib:** add missing types ([34deda5](https://github.com/angular/angular/commit/34deda5))
 * **test_lib:** reexport fake_async via angular/test ([687e7b5](https://github.com/angular/angular/commit/687e7b5))
 * **Typings:** Output public constructors in .d.ts files ([1926335](https://github.com/angular/angular/commit/1926335)), closes [#3926](https://github.com/angular/angular/issues/3926) [#3963](https://github.com/angular/angular/issues/3963)
-* **Typings:** Remove public exports added in 1926335b85af6c1fe56f4e36d0b95dcc92bb5c42 ([787d1f9](https://github.com/angular/angular/commit/787d1f9)), closes [#4147](https://github.com/angular/angular/issues/4147)
+* **Typings & Test API:**
+ - Remove public exports added in 1926335b85af6c1fe56f4e36d0b95dcc92bb5c42 ([787d1f9](https://github.com/angular/angular/commit/787d1f9))
+ - closes [#4147](https://github.com/angular/angular/issues/4147)
+ - BREAKING CHANGE: `RootTestComponent` is no longer `DebugElement`; to get to component instance use `testComp.debugElement.componentInstance`
 * **upgrade:** assert correct interleaving of evaluation. ([a562230](https://github.com/angular/angular/commit/a562230)), closes [#4436](https://github.com/angular/angular/issues/4436)
 * **WebWorker:** Add zone support to MessageBus ([f3da37c](https://github.com/angular/angular/commit/f3da37c)), closes [#4053](https://github.com/angular/angular/issues/4053)
 * **WebWorker:** Fix Todo Server demo and add test to ensure the demo can bootstrap. ([696edde](https://github.com/angular/angular/commit/696edde)), closes [#3970](https://github.com/angular/angular/issues/3970)