Fix race and limit access to mutator_thread_.

Fix race and limit access to mutator_thread_.

Unsynchronized access to the contents of another thread is dangerous, even for just ASSERTs, as shown by issue 23958.

Verify the bidirectional Isolate<->Thread pointer invariant when *setting* the mutator_thread_ field, and
limit the interface to comparisons with NULL and the current thread.

BUG=https://github.com/dart-lang/sdk/issues/23958
R=johnmccutchan@google.com

Committed: https://github.com/dart-lang/sdk/commit/7d82391e8949513ced7be0ee6cd56b9f4a38fcd8

[koda, 2015-08-04 20:24:57]

koda@google.com changed reviewers:
+ johnmccutchan@google.com

[Cutch, 2015-08-04 20:30:17]

lgtm

[koda, 2015-08-04 20:30:54]

Committed patchset #2 (id:20001) manually as
7d82391e8949513ced7be0ee6cd56b9f4a38fcd8 (presubmit successful).