Fix compilation error. ../../dart/runtime/vm/freelist.cc:126:32: warning: converting to non-pointer type ‘uword {aka long unsigned int}’ from NULL [-Wconversion-null]
Fix compilation error.
../../dart/runtime/vm/freelist.cc:126:32: warning: converting to non-pointer type ‘uword {aka long unsigned int}’ from NULL [-Wconversion-null]

R=zra@google.com

Review URL: https://codereview.chromium.org//150863006

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@32565 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk