Fix Android build warnings

runtime/vm/freelist_test.cc

 // Copyright (c) 2012, 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 "platform/assert.h"
 #include "vm/freelist.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
@@ skipping 141 matching lines @@
   delete[] objects;
 }
 
 
 TEST_CASE(FreeListProtectedVariableSizeObjects) {
   FreeList* free_list = new FreeList();
   const intptr_t kBlobSize = 8 * KB;
   const intptr_t kMinSize = 2 * kWordSize;
   uword* objects = new uword[kBlobSize / kMinSize];
   for (intptr_t i = 0; i < kBlobSize / kMinSize; ++i) {
-    objects[i] = NULL;
+    objects[i] = static_cast<uword>(NULL);
   }
 
   VirtualMemory* blob = VirtualMemory::ReserveAligned(kBlobSize, 4096);
   blob->Commit(/* is_executable = */ false);
   blob->Protect(VirtualMemory::kReadWrite);
 
   // Enqueue the large blob as one free block.
   free_list->Free(blob->start(), blob->size());
 
   // Write protect the whole region.
@@ skipping 14 matching lines @@
   e0 = Allocate(free_list, 3 * KB - 2 * kWordSize, true);
   ASSERT(e0);
 
   // Delete the memory associated with the test.
   delete blob;
   delete free_list;
   delete[] objects;
 }
 
 }  // namespace dart