Add a type for the `context` field in Dart's pre-generated change
detectors. This requires slight changes to set the dehydrated value of
`context` to `null` rather than `ChangeDetectionUtil.uninitialized()`,
which was its former dehydrated state.
Mirror these chagnes as closely as possible in the
`ChangeDetectionJITGenerator` to allow easier maintenance.
Closes#2070
Use the `ProtoViewDto` created by the render `Compiler` to create a
`ChangeDetectorDefinition`.
From there, generate a subclass of `AbstractChangeDetector` for each
`ChangeDetectorDefinition`.
Run some basic unit tests for the dynamic and JIT change detectors on
pre-generated change detectors.
Tim Blasi