Change the behavior of open on files to not truncate by default

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index 8f14be70dc0848764a4c3402c1e2ba0ed1a99b17..c66fe33ea5b97ce568c09c79518283aa6ae3b35c 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -105,8 +105,7 @@ static int ParseArguments(int argc,
 
 
 static void WriteSnapshotFile(const uint8_t* buffer, const intptr_t size) {
-  const bool kWritable = true;
-  File* file = File::Open(snapshot_filename, kWritable);
+  File* file = File::Open(snapshot_filename, File::kWriteTruncate);
   ASSERT(file != NULL);
   for (intptr_t i = 0; i < size; i++) {
     file->WriteByte(buffer[i]);