Use TypeSystem in the 'Extract Method' refactoring.

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 1050 matching lines @@
 main() {
   int a = 1 + 2;
 }
 ''');
     _createRefactoringForString('1 + 2');
     // do check
     await refactoring.checkInitialConditions();
     expect(refactoring.returnType, 'int');
   }
 
+  test_returnType_mixInterfaceFunction() async {
+    indexTestUnit('''
+main() {
+// start
+  if (true) {
+    return 1;
+  } else {
+    return () {};
+  }
+// end
+}
+''');
+    _createRefactoringForStartEndComments();
+    // do check
+    await refactoring.checkInitialConditions();
+    expect(refactoring.returnType, 'Object');
+  }
+
   test_returnType_statements() async {
     indexTestUnit('''
 main() {
 // start
   double v = 5.0;
 // end
   print(v);
 }
 ''');
     _createRefactoringForStartEndComments();
@@ skipping 1493 matching lines @@
 
 int res() {
   localFunction() {
     return 'abc';
   }
   return 42;
 }
 ''');
   }
 
+  test_statements_return_multiple_interfaceFunction() {
+    indexTestUnit('''
+main(bool b) {
+// start
+  if (b) {
+    return 1;
+  }
+  return () {};
+// end
+}
+''');
+    _createRefactoringForStartEndComments();
+    // apply refactoring
+    return _assertSuccessfulRefactoring('''
+main(bool b) {
+// start
+  return res(b);
+// end
+}
+
+Object res(bool b) {
+  if (b) {
+    return 1;
+  }
+  return () {};
+}
+''');
+  }
+
   test_statements_return_multiple_sameElementDifferentTypeArgs() {
     indexTestUnit('''
 main(bool b) {
 // start
   if (b) {
     print(true);
     return <int>[];
   } else {
     print(false);
     return <String>[];
@@ skipping 159 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();
   }
 }