[CP] Clear watcher subscriptions to avoid leaking package:watcher data

[srawlins]

Commit(s) to merge

5aee0f6

Target

stable

Prepared changelist for beta/stable

https://dart-review.googlesource.com/c/sdk/+/311466

Issue Description

Prior to this fix, there is a fairly significant memory leak in the Dart analyzer that increases used memory when edits to a pubspec.yaml file are made. This fix closes the leak.

What is the fix

The fix is to cancel subscriptions more aggresively.

Why cherry-pick

This memory leak represents a big area of complaint for existing users.

Risk

low

Issue link(s)

#52447

Extra Info

No response

[srawlins]

How we doin?

[mit-mit]

SGTM

[srawlins]

Thanks much!