Support symbol literals.

tests/compiler/dart2js_extra/private_symbol_literal_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test symbol literals with underscores.
+// These are currently unsupported by dart2js.
+
+library symbol_literal_test;
+
+main() {
+  print(#a);
+  print(#_a); /// 01: compile-time error
+
+  print(#a.b);
+  print(#_a.b); /// 02: compile-time error
+  print(#a._b); /// 03: compile-time error
+
+  print(#a.b.c);
+  print(#_a.b.c); /// 04: compile-time error
+  print(#a._b.c); /// 05: compile-time error
+  print(#a.b._c); /// 06: compile-time error
+}