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
529805508a