Special items in this block:
- checked_mode_helper is a helper library that needed extensive modifications. Fixing it up for Dart 2.0 meant simultaneously migrating
its users in blocks 113 and 163. Renamed it to dynamic_type_helper to try to reflect what it's actually doing now.
- Many minor breakouts of compile-time errors and conversion from static warnings.
BUG=
R=bkonyi@google.com
Review-Url: https://codereview.chromium.org/2999733002 .