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

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index 333d58707851d0c2316509b5912c7a2601b54a78..23bf45efa6ee0536a6dfbd31bc1d31247444ed7e 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -197,7 +197,8 @@ static Dart_Handle SetupRuntimeOptions(CommandLineOptions* options) {
 static void DumpPprofSymbolInfo() {
   if (generate_pprof_symbols_filename != NULL) {
     Dart_EnterScope();
-    File* pprof_file = File::Open(generate_pprof_symbols_filename, true);
+    File* pprof_file =
+        File::Open(generate_pprof_symbols_filename, File::kWriteTruncate);
     ASSERT(pprof_file != NULL);
     void* buffer;
     int buffer_size;