Improve propagation in binary expressions

pkg/analysis_server/test/services/refactoring/extract_method_test.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 test.services.refactoring.extract_method;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/correction/status.dart';
@@ skipping 1786 matching lines @@
 ''');
     _createRefactoringForString('v1 + v2 + v1');
     // apply refactoring
     return _assertSuccessfulRefactoring('''
 main() {
   var v1 = 1;
   var v2 = 2;
   var a = res(v1, v2); // marker
 }
 
-num res(int v1, int v2) => v1 + v2 + v1;
+int res(int v1, int v2) => v1 + v2 + v1;
 ''');
   }
 
   test_statements_assignment() {
     indexTestUnit('''
 main() {
   int v;
 // start
   v = 5;
 // end
@@ skipping 930 matching lines @@
    * Returns a deep copy of [refactoring] parameters.
    * There was a bug masked by updating parameter instances shared between the
    * refactoring and the test.
    */
   List<RefactoringMethodParameter> _getParametersCopy() {
     return refactoring.parameters.map((p) {
       return new RefactoringMethodParameter(p.kind, p.type, p.name, id: p.id);
     }).toList();
   }
 }