Show actualy FD for files in Observatory.

runtime/bin/file.cc

Index: runtime/bin/file.cc
diff --git a/runtime/bin/file.cc b/runtime/bin/file.cc
index e4548a51ec4903933bb6f3daa29a64d97660f5d5..a9b13fbc325ac63393e601cf5efa375ead0521b8 100644
--- a/runtime/bin/file.cc
+++ b/runtime/bin/file.cc
@@ -111,6 +111,13 @@ void FUNCTION_NAME(File_Close)(Dart_NativeArguments args) {
 }
 
 
+void FUNCTION_NAME(File_GetFD)(Dart_NativeArguments args) {
+  File* file = GetFilePointer(Dart_GetNativeArgument(args, 0));
+  ASSERT(file != NULL);
+  Dart_SetReturnValue(args, Dart_NewInteger(file->GetFD()));
+}
+
+
 void FUNCTION_NAME(File_ReadByte)(Dart_NativeArguments args) {
   File* file = GetFilePointer(Dart_GetNativeArgument(args, 0));
   ASSERT(file != NULL);