Make pub strong-mode clean.

lib/src/asset/dart/serialize/exception.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.
 
 import 'package:barback/barback.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import '../utils.dart';
 
 /// An exception that was originally raised in another isolate.
 ///
 /// Exception objects can't cross isolate boundaries in general, so this class
 /// wraps as much information as can be consistently serialized.
 class CrossIsolateException implements Exception {
   /// The name of the type of exception thrown.
   ///
   /// This is the return value of [error.runtimeType.toString()]. Keep in mind
   /// that objects in different libraries may have the same type name.
   final String type;
 
   /// The exception's message, or its [toString] if it didn't expose a `message`
   /// property.
-  final String message;
+  String get message => _message;
+  final String _message;
 
   /// The exception's stack chain, or `null` if no stack chain was available.
   final Chain stackTrace;
 
   /// Loads a [CrossIsolateException] from a serialized representation.
   ///
   /// [error] should be the result of [CrossIsolateException.serialize].
   CrossIsolateException.deserialize(Map error)
       : type = error['type'],
-        message = error['message'],
+        _message = error['message'],
         stackTrace = error['stack'] == null ? null :
             new Chain.parse(error['stack']);
 
   /// Serializes [error] to an object that can safely be passed across isolate
   /// boundaries.
   static Map serialize(error, [StackTrace stack]) {
     if (stack == null && error is Error) stack = error.stackTrace;
     return {
       'type': error.runtimeType.toString(),
       'message': getErrorMessage(error),
@@ skipping 47 matching lines @@
 ///
 /// This handles [AssetNotFoundException]s specially, ensuring that their
 /// metadata is preserved.
 CrossIsolateException deserializeException(Map error) {
   if (error['type'] == 'AssetNotFoundException') {
     return new _CrossIsolateAssetNotFoundException.deserialize(error);
   } else {
     return new CrossIsolateException.deserialize(error);
   }
 }