Fix windows compile warnings.

src/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3301 matching lines @@
     return symbol;
   // Now we know the length is 2, we might as well make use of that fact
   // when building the new string.
   } else if ((c1 | c2) <= String::kMaxAsciiCharCodeU) {  // We can do this
     ASSERT(IsPowerOf2(String::kMaxAsciiCharCodeU + 1));  // because of this.
     Object* result;
     { MaybeObject* maybe_result = heap->AllocateRawOneByteString(2);
       if (!maybe_result->ToObject(&result)) return maybe_result;
     }
     char* dest = SeqOneByteString::cast(result)->GetChars();
-    dest[0] = c1;
-    dest[1] = c2;
+    dest[0] = static_cast<char>(c1);
+    dest[1] = static_cast<char>(c2);
     return result;
   } else {
     Object* result;
     { MaybeObject* maybe_result = heap->AllocateRawTwoByteString(2);
       if (!maybe_result->ToObject(&result)) return maybe_result;
     }
     uc16* dest = SeqTwoByteString::cast(result)->GetChars();
     dest[0] = c1;
     dest[1] = c2;
     return result;
@@ skipping 4139 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   FIXED_ARRAY_SUB_INSTANCE_TYPE_LIST(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   memcpy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   memcpy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal