fix(testing): correctly import NgMatchers (#10077)

Some test cases incorrectly rely on the side effect of other test cases
importing `NgMatchers`. This commit fixes this by making `expect` in
`core/testing_internal` properly typed.

modules/@angular/core/test/di/reflective_injector_spec.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {expect} from '@angular/core/testing/testing_internal';
+import {expect} from '@angular/platform-browser/testing/matchers';
 
 import {BaseException} from '../../src/facade/exceptions';
 import {isBlank, isPresent, stringify} from '../../src/facade/lang';