Create class AST nodes.

dart/frog/tests/leg_only/src/BasicClassTest.dart

Index: dart/frog/tests/leg_only/src/BasicClassTest.dart
diff --git a/dart/frog/tests/leg_only/src/BasicClassTest.dart b/dart/frog/tests/leg_only/src/BasicClassTest.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c0e930814214407a93dfb7e37c54b515dc3e66f5
--- /dev/null
+++ b/dart/frog/tests/leg_only/src/BasicClassTest.dart
@@ -0,0 +1,27 @@
+// Copyright (c) 2011, 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.
+
+class A {
+}
+
+class SubA extends A {
+}
+
+class B {
+}
+
+void main() {
+  A a;
+  SubA subA;
+  B b;
+  a = a;
+  a = subA;
+  a = b; /// 01: static type error
+  subA = a;
+  subA = subA;
+  subA = b; /// 02: static type error
+  b = a; /// 03: static type error
+  b = subA; /// 04: static type error
+  b = b;
+}