Group together externally-managed UIDartState.
It's a frequent pattern to enable access to shell resources from the
framework via UIDartState. These handles are passed through several
other objects, where they lay mostly dormant. This refactor just aims
to ease maintenance of this state.
UIDartState is essentially just an easy to extend bus for dropping
off the shell kids in UIDartState, with constructors/operators that
remove the need for providing a full initializer lists in unittests.
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