[CFE] Incremental compiler has experimental invalidation strategy (first checkpoint)

This CL adds an option to the incremental compiler that enables an experiment
invalidation strategy that only rebuilds bodies for changed libraries (if possible).
As far as we know, for valid applications this should work fine, although,
applications with errors in them (or where the changed libraries have errors in them)
might not issue those errors correctly.

Known caveats:

- Doesn't work on dill library builders (aka if you load from dill we
  still have to invalidate transitively).
- Only body-in-brace-changes --- cannot add new private members for
  instance, nor changed fields.
- It doesn't track which bodies has changed so there's room for
  improvement in regards to for instance mixin stuff where we bail
  if a class from an invalidated file is used as a mixin.
- Offsets for methods, for instance, isn't updated
- Any errors issued at the outline stage won't be issued.

Change-Id: I6edf073ec47ea31429479ffe67e2e8b6d2cb80dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124980
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>