feat(test): add element probe

Usage: bootstrap the app with the special binding
`ELEMENT_PROBE_CONFIG` from `angular2/debug`.
This will provide a global method `ngProbe(element)` that
will expose a `DebugElement` with directive instances, ... on it.

During tests that use Angular's test injector, the probe is
enabled by default. The `DebugElement ` can be retrieved via the
function `inspectDomElement` of `angular2/debug`. Note
that the `TestComponentBuilder` already returns `DebugElement `s.

Closes #1992

modules/angular2/test/core/compiler/integration_spec.ts

@@ -725,9 +725,7 @@ export function main() {
                    var updateHost = injector.get(DirectiveUpdatingHostActions);
 
                    ObservableWrapper.subscribe(updateHost.setAttr, (_) => {
-                     expect(stringifyElement(domElement))
-                         .toEqual(
-                             '<div class="ng-binding" key="value" update-host-actions=""></div>');
+                     expect(DOM.hasAttribute(domElement, 'update-host-actions')).toBe(true);
                      async.done();
                    });