Fix clang build error

runtime/bin/main.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 <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 
 #include "include/dart_api.h"
 #include "include/dart_tools_api.h"
@@ skipping 1280 matching lines @@
                 uint8_t** output, 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 181 matching lines @@
   Platform::Exit(Process::GlobalExitCode());
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   dart::bin::main(argc, argv);
   UNREACHABLE();
 }