feat(test): allow tests to specify the platform and application providers used

With providers split into bundles, the test injector is now able to
use providers for a given bundle. Suggested provider lists for tests are
available in `angular2/platform/testing/<platform>`.

Change the providers for a test suite using `setBaseTestProviders`. This
should be done once at the start of the test suite, before any test cases
run.

BREAKING CHANGE: Tests are now required to use `setBaseTestProviders`
to set up. Assuming your tests are run on a browser, setup would change
as follows.

Before:

```js
// Somewhere in test setup
import {BrowserDomAdapter} from 'angular2/src/platform/browser/browser_adapter';
BrowserDomAdapter.makeCurrent
```

After:

```js
// Somewhere in the test setup
import {setBaseTestProviders} from 'angular2/testing';
import {
  TEST_BROWSER_PLATFORM_PROVIDERS,
  TEST_BROWSER_APPLICATION_PROVIDERS
} from 'angular2/platform/testing/browser';

setBaseTestProviders(TEST_BROWSER_PLATFORM_PROVIDERS,
                     TEST_BROWSER_APPLICATION_PROVIDERS);
```

Closes #5351, Closes #5585

Closes #5975

modules/angular2/src/platform/dom/dom_adapter.ts

@@ -23,6 +23,7 @@ export abstract class DomAdapter {
   abstract logGroup(error);
   abstract logGroupEnd();
 
+  /** @deprecated */
   abstract getXHR(): Type;
 
   /**