Fix warnings.

sdk/lib/_internal/pub/lib/src/dart.dart

 // Copyright (c) 2013, 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.
 
 /// A library for compiling Dart code and manipulating analyzer parse trees.
 library pub.dart;
 
 import 'dart:async';
 import 'dart:isolate';
 
@@ skipping 109 matching lines @@
   /// The exception's message, or its [toString] if it didn't expose a `message`
   /// property.
   final String message;
 
   /// The exception's stack trace, or `null` if no stack trace was available.
   final Trace stackTrace;
 
   /// Loads a [CrossIsolateException] from a serialized representation.
   ///
   /// [error] should be the result of [CrossIsolateException.serialize].
-  CrossIsolateException.deserialize(Object error)
-      : type = error['type'],
-        message = error['message'],
-        stackTrace = error['stack'] == null ? null :
+  factory CrossIsolateException.deserialize(Map error) {
+    var type = error['type'];
+    var message = error['message'];
+    var stackTrace = error['stack'] == null ? null :
             new Trace.parse(error['stack']);
+    return new CrossIsolateException._(type, message, stackTrace);
+  }
+
+  /// Loads a [CrossIsolateException] from a serialized representation.
+  ///
+  /// [error] should be the result of [CrossIsolateException.serialize].
+  CrossIsolateException._(this.type, this.message, this.stackTrace);
 
   /// Serializes [error] to an object that can safely be passed across isolate
   /// boundaries.
-  static Object serialize(error, [StackTrace stack]) {
+  static Map serialize(error, [StackTrace stack]) {
     if (stack == null) stack = getAttachedStackTrace(error);
     return {
       'type': error.runtimeType.toString(),
       'message': getErrorMessage(error),
       'stack': stack == null ? null : stack.toString()
     };
   }
 
   String toString() => "$message\n$stackTrace";
 }