Make summary format stable when adding getters to idl.

pkg/analyzer/lib/src/summary/base.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Base functionality which code generated summary classes are built upon.
  */
 library analyzer.src.summary.base;
 
 /**
+ * Annotation used in the summary IDL to indicate the id of a field.  The set
+ * of ids used by a class must cover the contiguous range from 0 to N-1, where
+ * N is the number of fields.
+ *
+ * In order to preserve forwards and backwards compatibility, id numbers must
+ * be stable between releases.  So when new fields are added they should take
+ * the next available id without renumbering other fields.
+ */
+class Id {
+  final int value;
+
+  const Id(this.value);
+}
+
+/**
  * Instances of this class represent data that has been read from a summary.
  */
 abstract class SummaryClass {
   /**
    * Translate the data in this class into a map whose keys are the names of
    * fields and whose values are the data stored in those fields.
    *
    * Intended for testing and debugging only.
    */
   Map<String, Object> toMap();
 }