Don't compute hashes in "--build-summary-exclude-informative" mode.

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 609 matching lines @@
    * [PackageBundleAssembler.currentMinorVersion].
    */
   @Id(6)
   int get minorVersion;
 
   /**
    * List of MD5 hashes of the files listed in [unlinkedUnitUris].  Each hash
    * is encoded as a hexadecimal string using lower case letters.
    */
   @Id(4)
+  @informative
   List<String> get unlinkedUnitHashes;
 
   /**
    * Unlinked information for the compilation units constituting the package.
    */
   @Id(2)
   List<UnlinkedUnit> get unlinkedUnits;
 
   /**
    * The list of URIs of items in [unlinkedUnits], e.g. `dart:core/bool.dart`.
@@ skipping 2044 matching lines @@
    */
   @Id(11)
   int get visibleLength;
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   @Id(12)
   int get visibleOffset;
 }