Speed up gen_snapshot by writing all bytes at once.

runtime/bin/gen_snapshot.cc

 // Copyright (c) 2013, 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.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 125 matching lines @@
     return -1;
   }
 
   return 0;
 }
 
 
 static void WriteSnapshotFile(const uint8_t* buffer, const intptr_t size) {
   File* file = File::Open(snapshot_filename, File::kWriteTruncate);
   ASSERT(file != NULL);
-  for (intptr_t i = 0; i < size; i++) {
-    file->WriteByte(buffer[i]);
+  if (!file->WriteFully(buffer, size)) {
+    Log::PrintErr("Error: Failed to write full snapshot.\n\n");
   }
   delete file;
 }
 
 
 class UriResolverIsolateScope {
  public:
   UriResolverIsolateScope() {
     ASSERT(isolate != NULL);
     snapshotted_isolate_ = Dart_CurrentIsolate();
@@ skipping 407 matching lines @@
   }
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }