Prepare for decoupling analyzer ASTs from element model.

pkg/analyzer/lib/dart/ast/resolution_base_classes.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.
+
+/**
+ * Abstract base class for a resolved element maintained in an AST data
+ * structure.
+ *
+ * This abstract type decouples the AST representation from depending on the
+ * element model.
+ */
+abstract class ResolutionTarget {}
+
+/**
+ * Abstract base class for a resolved type maintained in AST data structure.
+ *
+ * This abstract type decouples the AST representation from depending on the
+ * type model.
+ */
+abstract class ResolutionType {}