[CP][stable channel] Fix for VM crash with type mis-match https://github.com/dart-lang/sdk/issues/43464

[a-siva]

commit(s) to merge: 68fd2a9

merge instructions: clean merge

What is the issue: type assumption could lead to unsound optimizations. This leads to program crashes when flutter users are developing applications.

What is the fix: The code was reworked to make pessimistic type assumptions that will not lead to unsound optimizations.

Why cherrypick: Flutter users can hit this bug which developing applications resulting in crashes that could be hard to diagnose down to this issue, the fix itself is small and safe.

Risk: Low risk, patch is self-contained as validated for some time.

Link to original issue(s): #43464

/cc @kevmoo @mit-mit @whesse @athomas @vsmenon @franklinyow

[vsmenon]

Have we validated that this commit rolled successfully into Flutter master and into g3?

[a-siva]

The change has rolled into flutter and google3 (the last google3 roll done today was with dart version 9560a32779 which includes this change

[vsmenon]

lgtm

[franklinyow]

Approved