Address MSVS 2013 warning C4146: unary minus operator applied to unsigned type, result still unsign…

runtime/vm/object_set.h

Index: runtime/vm/object_set.h
diff --git a/runtime/vm/object_set.h b/runtime/vm/object_set.h
index 6f687a9663af987907ae5376147e8e83dffbf09f..cdc642f518b0f05ae2582a8f6e8123f802c411f9 100644
--- a/runtime/vm/object_set.h
+++ b/runtime/vm/object_set.h
@@ -31,8 +31,10 @@ class ObjectSet {
     ASSERT(start_ <= end_);
     size_ = SizeFor((end_ - start_) >> kWordSizeLog2);
     allocation_ = new uword[size_];
-    data_ = &allocation_[-((start >> kWordSizeLog2) / kBitsPerWord)];
-    ASSERT(allocation_ == &data_[(start >> kWordSizeLog2) / kBitsPerWord]);
+    const intptr_t skipped_bitfield_words =
+        (start >> kWordSizeLog2) / kBitsPerWord;
+    data_ = &allocation_[-skipped_bitfield_words];
+    ASSERT(allocation_ == &data_[skipped_bitfield_words]);
     Clear();
   }