Use TypeSystem in the 'Extract Method' refactoring.

pkg/analysis_server/lib/src/services/refactoring/extract_method.dart

Index: pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
diff --git a/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart b/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
index da79930df9a411e83763e1be010e622f8f932ecd..e83082b54c262d65ca6da4146e584a5c5ddba81d 100644
--- a/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
+++ b/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
@@ -594,7 +594,7 @@ class ExtractMethodRefactoringImpl extends RefactoringImpl
     }
     // maybe ends with "return" statement
     if (_selectionStatements != null) {
-      _ReturnTypeComputer returnTypeComputer = new _ReturnTypeComputer();
+      _ReturnTypeComputer returnTypeComputer = new _ReturnTypeComputer(context);
       _selectionStatements.forEach((statement) {
         statement.accept(returnTypeComputer);
       });
@@ -1158,8 +1158,12 @@ class _Occurrence {
 }
 
 class _ReturnTypeComputer extends RecursiveAstVisitor {
+  final AnalysisContext context;
+
   DartType returnType;
 
+  _ReturnTypeComputer(this.context);
+
   @override
   visitBlockFunctionBody(BlockFunctionBody node) {}
 
@@ -1182,7 +1186,8 @@ class _ReturnTypeComputer extends RecursiveAstVisitor {
       if (returnType is InterfaceType && type is InterfaceType) {
         returnType = InterfaceType.getSmartLeastUpperBound(returnType, type);
       } else {
-        returnType = returnType.getLeastUpperBound(type);
+        returnType = context.typeSystem
+            .getLeastUpperBound(context.typeProvider, returnType, type);
       }
     }
   }