Logical compound assignment operators &&= and ||=

Cf. issues https://github.com/dart-lang/sdk/issues/26996, meta, and https://github.com/dart-lang/sdk/issues/26997, analyzer, (there are others, but they should probably be replaced by a single front-end issue), the compound assignment operators `&&=` and `||=` have been in the pipeline for a full specification and an implementation for a long time.

We ought to find a placement for this task in terms of milestones and priorities, or drop it.