Fix a type cast exception in the create-constructor fix (issue 42830)

Change-Id: I47b11000c2c4ec96856a58d43f9b7b6493cc7908
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155825
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: bwilkerson

pkg/analysis_server/lib/src/services/correction/dart/create_constructor.dart

@@ -65,10 +65,10 @@ class CreateConstructor extends CorrectionProducer {
     // prepare target ClassDeclaration
     ClassElement targetElement = targetType.element;
     var targetResult = await sessionHelper.getElementDeclaration(targetElement);
-    if (targetResult.node is! ClassOrMixinDeclaration) {
+    var targetNode = targetResult.node;
+    if (targetNode is! ClassDeclaration) {
       return;
     }
-    ClassOrMixinDeclaration targetNode = targetResult.node;
 
     // prepare location
     var targetLocation = CorrectionUtils(targetResult.resolvedUnit)
@@ -108,10 +108,10 @@ class CreateConstructor extends CorrectionProducer {
     // prepare target ClassDeclaration
     var targetElement = constructorElement.enclosingElement;
     var targetResult = await sessionHelper.getElementDeclaration(targetElement);
-    if (targetResult.node is! ClassOrMixinDeclaration) {
+    var targetNode = targetResult.node;
+    if (targetNode is! ClassDeclaration) {
       return;
     }
-    ClassOrMixinDeclaration targetNode = targetResult.node;
 
     // prepare location
     var targetLocation = CorrectionUtils(targetResult.resolvedUnit)

pkg/analysis_server/test/src/services/correction/fix/create_constructor_test.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analysis_server/src/services/correction/fix.dart';
+import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ -104,6 +105,19 @@ main() {
 ''');
   }
 
+  Future<void> test_mixin() async {
+    verifyNoTestUnitErrors = false;
+    await resolveTestUnit('''
+mixin M {}
+void f() {
+  new M(3);
+}
+''');
+    await assertNoFix(
+        errorFilter: (error) =>
+            error.errorCode != CompileTimeErrorCode.MIXIN_INSTANTIATE);
+  }
+
   Future<void> test_named() async {
     await resolveTestUnit('''
 class A {