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
Tobias Bosch