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_lib.ts

@@ -1,3 +1,5 @@
 export * from './src/test_lib/test_lib';
 export * from './src/test_lib/utils';
 export * from './src/test_lib/fake_async';
+export * from './src/test_lib/test_component_builder';
+export * from './debug';