[flutter tools] Add a DelegatingLogger class

Move most of `DelegateLogger` `from test/src/testbed.dart` to
`lib/src/base/logger.dart` to better formalize the common practice of
chaining `Logger`s together.  I renamed the class since it isn't
itself the delegate and to better match the `Delegating...` classes
from `package:collection`.

Additionally, add a freestanding `asLogger<T>` function to "cast" a
`Logger` into a matching delegate if possible.  This will allow
`Logger` chains to be ordered a *bit* more freely (e.g.
`NotifyingLogger` and `AppRunLogger` will no longer required to be
at the end of the chain, an unwritten rule that has led to breakage in
google3).  Chain order still matters since lack of virtual dispatch
means that parent `Logger`s can never invoke child methods, however.

I made `asLogger<T>` a freestanding function because I didn't want to
make it part of the `Logger` interface (and I thought that making it
an extension method might be weird).

Bonus cleanup:
There no longer appears to be a way to construct an `AppRunLogger`
with a null parent, so remove all of code paths for that case and
make the `parent` construction parameter required.