hide Symbol exported from dart:_internal (flutter#3861)

Both dart:core and dart:_internal export Symbol. 

This is silently ignored by the VM but some tools (e.g. fasta) report a warning when processing dart:ui sources.

See dart-lang/sdk#30127 for more details.

lib/ui/ui.dart

@@ -11,7 +11,7 @@
 /// text, layout, and rendering subsystems.
 library dart.ui;
 
-import 'dart:_internal';
+import 'dart:_internal' hide Symbol;
 import 'dart:async';
 import 'dart:convert';
 import 'dart:developer' as developer;