Add file modes for opening a file write only

runtime/bin/file.cc

Index: runtime/bin/file.cc
diff --git a/runtime/bin/file.cc b/runtime/bin/file.cc
index 15088caab93cf996bae2d9fe8d1e9ee7f7925bf3..59f8300a266fb602b639043a82ebd4f2e0ed2980 100644
--- a/runtime/bin/file.cc
+++ b/runtime/bin/file.cc
@@ -59,13 +59,21 @@ bool File::WriteFully(const void* buffer, int64_t num_bytes) {
 File::FileOpenMode File::DartModeToFileMode(DartFileOpenMode mode) {
   ASSERT(mode == File::kDartRead ||
          mode == File::kDartWrite ||
-         mode == File::kDartAppend);
+         mode == File::kDartAppend ||
+         mode == File::kDartWriteOnly ||
+         mode == File::kDartWriteOnlyAppend);
   if (mode == File::kDartWrite) {
     return File::kWriteTruncate;
   }
   if (mode == File::kDartAppend) {
     return File::kWrite;
   }
+  if (mode == File::kDartWriteOnly) {
+    return File::kWriteOnlyTruncate;
+  }
+  if (mode == File::kDartWriteOnlyAppend) {
+    return File::kWriteOnly;
+  }
   return File::kRead;
 }