Fix compilation error.

runtime/vm/freelist.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/freelist.h"
 
 #include <map>
 #include <utility>
 
 #include "vm/bit_set.h"
@@ skipping 105 matching lines @@
                                    VirtualMemory::kReadWrite /*Execute*/);
         ASSERT(status);
       }
       if (previous == NULL) {
         free_lists_[kNumLists] = current->next();
       } else {
         // If the previous free list element's next field is protected, it
         // needs to be unprotected before storing to it and reprotected
         // after.
         bool target_is_protected = false;
-        uword target_address = NULL;
+        uword target_address = 0L;
         if (is_protected) {
           uword writable_start = reinterpret_cast<uword>(current);
           uword writable_end =
               writable_start + size + FreeListElement::kHeaderSize - 1;
           target_address = previous->next_address();
           target_is_protected =
               !VirtualMemory::InSamePage(target_address, writable_start) &&
               !VirtualMemory::InSamePage(target_address, writable_end);
         }
         if (target_is_protected) {
@@ skipping 174 matching lines @@
       !VirtualMemory::InSamePage(remainder_address - 1, remainder_address)) {
     bool status =
         VirtualMemory::Protect(reinterpret_cast<void*>(remainder_address),
                                remainder_size,
                                VirtualMemory::kReadExecute);
     ASSERT(status);
   }
 }
 
 }  // namespace dart