Add an "api signature" to summaries.

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

 // Copyright (c) 2015, 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.
 
 /**
  * This file is an "idl" style description of the summary format.  It
  * contains abstract classes which declare the interface for reading data from
  * summaries.  It is parsed and transformed into code that implements the
  * summary format.
  *
@@ skipping 632 matching lines @@
 
 /**
  * Summary information about a package.
  */
 @TopLevel('PBdl')
 abstract class PackageBundle extends base.SummaryClass {
   factory PackageBundle.fromBuffer(List<int> buffer) =>
       generated.readPackageBundle(buffer);
 
   /**
+   * MD5 hash of the non-informative fields of the [PackageBundle] (not
+   * including this one).  This can be used to identify when the API of a
+   * package may have changed.
+   */
+  @Id(7)
+  String get apiSignature;
+
+  /**
    * Linked libraries.
    */
   @Id(0)
   List<LinkedLibrary> get linkedLibraries;
 
   /**
    * The list of URIs of items in [linkedLibraries], e.g. `dart:core` or
    * `package:foo/bar.dart`.
    */
   @Id(1)
@@ skipping 2122 matching lines @@
    */
   @Id(11)
   int get visibleLength;
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   @Id(12)
   int get visibleOffset;
 }