refactor(testing): move common testing logic into test_injector

Before, all test framework wrappers (internal for dart and js/ts,
angular2_test for dart and testing for js/ts) had similar logic to
keep track of current global test injector and test provider list.
This change wraps that logic into one class managed by the test
injector.

Closes #5920

modules/angular2/test/web_workers/shared/message_bus_spec.ts

@@ -5,7 +5,6 @@ import {
   it,
   expect,
   beforeEach,
-  createTestInjectorWithRuntimeCompiler,
   beforeEachProviders,
   SpyObject,
   proxy

modules/angular2/test/web_workers/shared/service_message_broker_spec.ts

@@ -5,7 +5,6 @@ import {
   it,
   expect,
   beforeEach,
-  createTestInjectorWithRuntimeCompiler,
   beforeEachProviders,
   SpyObject,
   proxy