Fix for queue closed crash

Fix for queue closed crash

It is possible for JavaScriptBackend.onQueueEmpty to be called twice, first time with isTreeShakingDisabled false (hence calling Enqueuer.enqueueReflectiveStaticFields), the second time with isTreeShakingDisabled true (hence calling Enqueuer.enqueueEverything).  The crash is caused by using the same flag to avoid repeating work for enqueueReflectiveStaticFields and enqueueEverything.

The fix is to split the flag so we can enqueue statics and later enqueue everything.

R=floitsch@google.com, johnniwinther@google.com

Committed: https://code.google.com/p/dart/source/detail?r=34374

[sra1, 2014-03-25 01:47:31]

What I have not figured out is how to reproduce this in a small example.  Peter,
do you have any ideas on this?

[ahe, 2014-03-25 09:59:27]

I think Florian made these changes.

I don't understand why enqueueReflectiveStaticFields has a guard.

[Johnni Winther, 2014-03-25 10:02:57]

lgtm

[floitsch, 2014-03-25 10:13:21]

LGTM.

minimal example:
===
@MirrorsUsed(targets: const ["foo"])
import 'dart:mirrors';

final foo = reflect(reflect(9)).getField(#getField);

void main() {}
===

[sra1, 2014-03-25 17:33:54]

Committed patchset #2 manually as r34374 (presubmit successful).