At shutdown, remove isolate from list before tearing it down.
At shutdown, remove isolate from list before tearing it down.

This avoid exposing the decaying, possibly illegal, state of the isolate to isolate visitors.

BUG=dart:19219
R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org//327123002

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@37211 260f80e4-7a28-3924-810f-c04153c831b5

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