Use a better way to compute InterfaceTypes least upper bound.

pkg/analysis_server/test/services/refactoring/extract_method_test.dart

Index: pkg/analysis_server/test/services/refactoring/extract_method_test.dart
diff --git a/pkg/analysis_server/test/services/refactoring/extract_method_test.dart b/pkg/analysis_server/test/services/refactoring/extract_method_test.dart
index c21cbb8c03e5ec6c14ea1cd732708a161f7db6d3..6ecc3a0cb692136e4897cd3c482dc47a67f94283 100644
--- a/pkg/analysis_server/test/services/refactoring/extract_method_test.dart
+++ b/pkg/analysis_server/test/services/refactoring/extract_method_test.dart
@@ -2278,6 +2278,37 @@ int res() {
 ''');
   }
 
+  test_statements_return_multiple_sameElementDifferentTypeArgs() {
+    indexTestUnit('''
+main(bool b) {
+// start
+  if (b) {
+    return <int>[];
+  } else {
+    return <String>[];
+  }
+// end
+}
+''');
+    _createRefactoringForStartEndComments();
+    // apply refactoring
+    return _assertSuccessfulRefactoring('''
+main(bool b) {
+// start
+  return res(b);
+// end
+}
+
+List res(bool b) {
+  if (b) {
+    return <int>[];
+  } else {
+    return <String>[];
+  }
+}
+''');
+  }
+
   test_statements_return_single() {
     indexTestUnit('''
 main() {