Fix clang build error

runtime/bin/vmservice_dartium.cc

 // Copyright (c) 2015, 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 "bin/vmservice_dartium.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 #include "bin/eventhandler.h"
 #include "bin/platform.h"
@@ skipping 88 matching lines @@
                                        unsigned int* output_length) {
   ASSERT(input != NULL);
   ASSERT(input_len > 0);
   ASSERT(output != NULL);
   ASSERT(output_length != NULL);
 
   // Initialize output.
   *output = NULL;
   *output_length = 0;
 
-  const intptr_t kChunkSize = 256 * 1024;
+  const unsigned int kChunkSize = 256 * 1024;
   uint8_t chunk_out[kChunkSize];
   z_stream strm;
   strm.zalloc = Z_NULL;
   strm.zfree = Z_NULL;
   strm.opaque = Z_NULL;
   strm.avail_in = 0;
   strm.next_in = 0;
   int ret = inflateInit2(&strm, 32 + MAX_WBITS);
   ASSERT(ret == Z_OK);
 
@@ skipping 36 matching lines @@
 }
 
 /* DISALLOW_ALLOCATION */
 void VmServiceServer::operator delete(void* pointer)  {
   fprintf(stderr, "unreachable code\n");
   abort();
 }
 
 }  // namespace bin
 }  // namespace dart