feat(tests): manage asynchronous tests using zones

Instead of using injectAsync and returning a promise, use the `async` function
to wrap tests. This will run the test inside a zone which does not complete
the test until all asynchronous tasks have been completed.

`async` may be used with the `inject` function, or separately.

BREAKING CHANGE:

`injectAsync` is now deprecated. Instead, use the `async` function
to wrap any asynchronous tests.

Before:
```
it('should wait for returned promises', injectAsync([FancyService], (service) => {
  return service.getAsyncValue().then((value) => { expect(value).toEqual('async value'); });
}));

it('should wait for returned promises', injectAsync([], () => {
  return somePromise.then(() => { expect(true).toEqual(true); });
}));
```

After:
```
it('should wait for returned promises', async(inject([FancyService], (service) => {
  service.getAsyncValue().then((value) => { expect(value).toEqual('async value'); });
})));

// Note that if there is no injection, we no longer need `inject` OR `injectAsync`.
it('should wait for returned promises', async(() => {
  somePromise.then() => { expect(true).toEqual(true); });
}));
```

Closes #7735

modules/angular2/test/testing/testing_public_browser_spec.dart

@@ -0,0 +1,7 @@
+library angular2.test.testing.testing_browser_pec;
+
+/**
+ * This is intentionally left blank. The public test lib is only for TS/JS
+ * apps.
+ */
+main() {}