Add an "api signature" to summaries.

pkg/analyzer/lib/src/summary/format.fbs

 // 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 has been automatically generated.  Please do not edit it manually.
 // To regenerate the file, use the script "pkg/analyzer/tool/generate_files".
 
 
 /**
  * Kind of a source in the cache.
@@ skipping 1163 matching lines @@
    * compilation unit with propagated and inferred types.
    */
   types:[EntityRef] (id: 1);
 }
 
 /**
  * Summary information about a package.
  */
 table PackageBundle {
   /**
+   * 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.
+   */
+  apiSignature:string (id: 7);
+
+  /**
    * Linked libraries.
    */
   linkedLibraries:[LinkedLibrary] (id: 0);
 
   /**
    * The list of URIs of items in [linkedLibraries], e.g. `dart:core` or
    * `package:foo/bar.dart`.
    */
   linkedLibraryUris:[string] (id: 1);
 
@@ skipping 1243 matching lines @@
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   visibleOffset:uint (id: 12);
 }
 
 root_type PackageBundle;
 
 file_identifier "PBdl";