Tests which post messages to the window can cause failures

The `data` attribute on the message here: https://github.com/dart-lang/test/blob/4940116222e305f99b7504e8a6c7ab5f769aedaa/tool/host.dart#L208 is dynamic and therefore can be any type. If a test is using the message channel it can interfere with the test infrastructure. We should make the messages coming from the test infrastructure easily identifiable.

We likely need to update the logic here along with similar calls:
https://github.com/dart-lang/test/blob/ad35c5c1c47aeb526e62a65d5db5bc285cca0a4f/lib/dart.js#L14

Start by comparing the message handling in tool/host.dart around line 208 with the similar calls in lib/dart.js around line 14. Trace how the test infrastructure posts messages through the window channel, then make those messages identifiable without colliding with arbitrary test data.

Written by the indexing model from the issue text.