[kernel] CloneVisitor should preserve isDefault on switch cases.

Fixes https://github.com/dart-lang/sdk/issues/32971

Change-Id: I16bf688080ee60853f5de449ead3e7feaec37fc2
Reviewed-on: https://dart-review.googlesource.com/52602
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

pkg/kernel/lib/clone.dart

@@ -345,7 +345,8 @@ class CloneVisitor implements TreeVisitor {
       switchCases[switchCase] = new SwitchCase(
           switchCase.expressions.map(clone).toList(),
           new List<int>.from(switchCase.expressionOffsets),
-          null);
+          null,
+          isDefault: switchCase.isDefault);
     }
     return new SwitchStatement(
         clone(node.expression), node.cases.map(clone).toList());

tests/language_2/vm/regress_32971_test.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2018, 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.
+
+// Test verifying that default switch cast is cloned correctly by the
+// mixin transformation.
+
+import "package:expect/expect.dart";
+
+void main() {
+  final o = new A();
+  Expect.isTrue(o.f());
+  Expect.isTrue(o.g());
+}
+
+class A extends B with M {}
+
+class B {
+  bool f() {
+    switch (true) {
+      default:
+        return true;
+    }
+    return false;
+  }
+}
+
+class M {
+  bool g() {
+    switch (true) {
+      default:
+        return true;
+    }
+    return false;
+  }
+}