Add navigation support in plugins

pkg/analyzer_plugin/lib/utilities/pair.dart

+// Copyright (c) 2017, 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.
+
+/**
+ * A pair of values.
+ */
+class Pair<E, F> {
+  final E first;
+  final F last;
+
+  Pair(this.first, this.last);
+
+  @override
+  int get hashCode => first.hashCode ^ last.hashCode;
+
+  @override
+  bool operator ==(other) {
+    if (other is! Pair) {
+      return false;
+    }
+    return other.first == first && other.last == last;
+  }
+
+  @override
+  String toString() => '($first, $last)';
+}