b7ae764479
[observatory] Display WeakReference targets. TEST=manually look at inspector.dart under Observatory Change-Id: Ibfcc266528e28afdb9876335c0dfac94b178c1c5 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/269982 Reviewed-by: Ben Konyi <bkonyi@google.com> Commit-Queue: Ryan Macnak <rmacnak@google.com>