Add the CHECK_ISOLATE macro, which validates that the current isolate

Add some checking macros:

CHECK_ISOLATE -- make sure that there is a current isolate.
CHECK_NO_ISOLATE -- the opposite.
CHECK_ISOLATE_SCOPE -- make sure that there is a current isolate and it has an API scope.

DARTSCOPE changes to call CHECK_ISOLATE_SCOPE.  If you don't want
that, use DARTSCOPE_NOCHECKS.

Went through all of the DART_EXPORT function and made sure they all
had the appropriate checking.

Rewrite DARTSCOPE a bit to avoid evaluating the macro param multiple
times.
Committed: https://code.google.com/p/dart/source/detail?r=1990

[siva, 2011-11-30 01:11:59]

lgtm

[turnidge, 2011-11-30 19:23:54]

Made some changes.  PTAL.

[siva, 2011-12-01 00:17:15]

lgtm

[turnidge, 2011-12-01 00:44:42]

(adding antonm to keep him in the loop)

[Anton Muhin, 2011-12-01 01:17:10]

Nice stuff.  If we break, we're ourselves to blame :)