Use a better way to compute InterfaceTypes least upper bound.

pkg/analysis_server/lib/src/services/refactoring/extract_method.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.
 
 library services.src.refactoring.extract_method;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
 import 'package:analysis_server/src/services/correction/name_suggestion.dart';
@@ skipping 1095 matching lines @@
   }
 
   @override
   visitReturnStatement(ReturnStatement node) {
     Expression expression = node.expression;
     if (expression != null) {
       DartType type = expression.bestType;
       if (returnType == null) {
         returnType = type;
       } else {
-        returnType = returnType.getLeastUpperBound(type);
+        if (returnType is InterfaceType && type is InterfaceType) {
+          returnType = InterfaceType.getSmartLeastUpperBound(returnType, type);
+        } else {
+          returnType = returnType.getLeastUpperBound(type);
+        }
       }
     }
   }
 }
 
 
 /**
  * Generalized version of some source, in which references to the specific
  * variables are replaced with pattern variables, with back mapping from the
  * pattern to the original variable names.
  */
 class _SourcePattern {
   String patternSource;
   Map<String, String> originalToPatternNames = {};
 }