feat(NgZone): isStable

[PatrickJS]

• Please check if the PR fulfills these requirements
• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  feature
• What is the current behavior? (You can also link to an open issue here)
  you have to use _isStable
• What is the new behavior (if this is a feature change)?
  you can use the getter isStable
• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  nope
• Other information:
  I use _isStable for both the server and requestIdleCallback polyfill

[mhevery]

Looks like you forgot to run lint. Also could you add test.

[PatrickJS]

oops, ya totes

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