Fix warnings.

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

Index: sdk/lib/_internal/pub/lib/src/dart.dart
diff --git a/sdk/lib/_internal/pub/lib/src/dart.dart b/sdk/lib/_internal/pub/lib/src/dart.dart
index b7765b81475c563adc9419c947e69ba4a8818c2a..f6d8b11074aa9f71f942e6434db7b0e698f7aa2c 100644
--- a/sdk/lib/_internal/pub/lib/src/dart.dart
+++ b/sdk/lib/_internal/pub/lib/src/dart.dart
@@ -127,15 +127,22 @@ class CrossIsolateException implements Exception {
   /// 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(),