This doesn't include the copious changes in collection_patch.dart
because I believe (1) those are quite old and (2) are dart2js specific.
The changes here have basically no effect on DDC, but it at least gets
us a little better test coverage and reduces noise in the diffs.
R=sra@google.com
Review URL: https://codereview.chromium.org/1977003002 .
Bob Nystrom