Lint to detect dangling identifiers in docs (#240).

test/rules/comment_references.dart

+// Copyright (c) 2016, 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 w/ `dart test/util/solo_test.dart comment_references`
+
+/// Writes [y]. #LINT
+void write(int x) {}
+
+/// [String] is OK.
+class A {
+  /// But [zap] is not. #LINT
+  int z;
+
+  /// Reads [x] and assigns to [z]. #OK
+  void read(int x) {}
+
+  /// Writes [y]. #LINT
+  void write(int x) {}
+}