Fix clang unused-private-field error.

Fix clang unused-private-field error.

../../runtime/vm/dart_api_state.h:736:18: error: private field 'key_' is not used [-Werror,-Wunused-private-field]
  ThreadLocalKey key_;
                   ^
R=scheglov@google.com
TEST=ninja -C out/Debug
Signed-off-by: Thiago Farina <tfarina@chromium.org>;

Committed: https://code.google.com/p/dart/source/detail?r=21778

[tfarina, 2013-04-19 13:24:03]

Hi, could you review this to me?

Note that I don't have dart commit access.

Thanks,

[tfarina, 2013-04-19 22:13:28]

Egorov, looks like someone assigned you as a reviewer instead of
scheglov@google.com.

May be could you review this to me instead?

Thanks,

[Vyacheslav Egorov (Google), 2013-04-19 22:31:47]

LGTM

[Vyacheslav Egorov (Google), 2013-04-19 22:42:59]

Committed patchset #1 manually as r21778 (presubmit successful).