The test framework previously made it impossible to actually catch all exceptions from the message channels system, because it "forked" the future chain and didn't catch exceptions on its end of the fork. This catches the exceptions on the forked branch. Originally I tried flattening the chain but that causes Zone issues in some tests with FakeAsync.

This unblocks flutter/packages#265 which, somehow, I don't even remember how, is part of the yak shave to fixing #64830.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.