Add file modes for opening a file write only

runtime/bin/file.h

Index: runtime/bin/file.h
diff --git a/runtime/bin/file.h b/runtime/bin/file.h
index 7a8107fd91f4f16629aec759ee9ba50e46aa1281..633c006175fa77f3f081815644e2b4b85f2a5b3e 100644
--- a/runtime/bin/file.h
+++ b/runtime/bin/file.h
@@ -26,15 +26,20 @@ class File {
     kRead = 0,
     kWrite = 1,
     kTruncate = 1 << 2,
-    kWriteTruncate = kWrite | kTruncate
+    kWriteOnly = 1 << 3,
+    kWriteTruncate = kWrite | kTruncate,
+    kWriteOnlyTruncate = kWriteOnly | kTruncate
   };
 
   // These values have to be kept in sync with the mode values of
-  // FileMode.READ, FileMode.WRITE and FileMode.APPEND in file.dart.
+  // FileMode.READ, FileMode.WRITE, FileMode.APPEND,
+  // FileMode.WRITE_ONLY and FileMode.WRITE_ONLY_APPEND in file.dart.
   enum DartFileOpenMode {
     kDartRead = 0,
     kDartWrite = 1,
-    kDartAppend = 2
+    kDartAppend = 2,
+    kDartWriteOnly = 3,
+    kDartWriteOnlyAppend = 4
   };
 
   enum Type {