Avoid race in isolate shutdown; add assertions, error messages

The sweeper must not be running during isolate shutdown.
In release mode, there seems to be nothing to prevent this.
In debug mode, the "Verify" call waits for the sweeper, but there is still a race between the task count update and the ExitIsolateAsHelper call, which could cause problems.

Fix both of these, and add more assertions and verbose error messages.

- make sweeper task cleanly exit isolate *before* notifying
- wait for sweeper before shutting down isolate
- verbose pthread failures

BUG=
R=asiva@google.com

Committed: https://github.com/dart-lang/sdk/commit/868d2c6c3e949a03c395efc2c3bfbe0761a295dc

[koda, 2015-07-13 19:47:16]

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

[koda, 2015-07-13 19:47:17]

I'll make identical changes on mac/android if you think this looks good.

[koda, 2015-07-13 22:35:07]

On 2015/07/13 19:47:17, koda wrote:
> I'll make identical changes on mac/android if you think this looks good.

Updated PTAL

[siva, 2015-07-13 23:37:00]

lgtm

https://codereview.chromium.org/1233563004/diff/20001/runtime/vm/isolate.cc
File runtime/vm/isolate.cc (right):

https://codereview.chromium.org/1233563004/diff/20001/runtime/vm/isolate.cc#n...
runtime/vm/isolate.cc:1442: }
I am wondering if we should do this wait after removing the isolate from the
list of isolates.

[koda, 2015-07-14 00:41:40]

https://codereview.chromium.org/1233563004/diff/20001/runtime/vm/isolate.cc
File runtime/vm/isolate.cc (right):

https://codereview.chromium.org/1233563004/diff/20001/runtime/vm/isolate.cc#n...
runtime/vm/isolate.cc:1442: }
On 2015/07/13 23:37:00, siva wrote:
> I am wondering if we should do this wait after removing the isolate from the
> list of isolates.

Done.

[koda, 2015-07-14 00:49:54]

Committed patchset #3 (id:40001) manually as
868d2c6c3e949a03c395efc2c3bfbe0761a295dc (presubmit successful).