optional Inject.get and .getByKey (or fallback value without throwing exception)

[isoos]

Injector.get(Type) and .getByKey(Key) will throw an exception if the key is not bound. If the component depends on many optional configuration item, wrapping these calls into try-catch blocks is about 50-60x slower than just returning a null value or calling a fallback function.

Please add support for having optional dependencies e.g. by following the num.parse() or the Iterable.firstWhere() method signature for fallbacks.