Migrate language block 46 - canonical_const ... char_escape.

tests/language/case_expression_with_assignment_test.dart

-// Copyright (c) 2017, 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.
-
-// Regression test for crash in VM parser (issue 29370).
-
-import "package:expect/expect.dart";
-
-const ERROR_A = 0;
-const ERROR_B = 1;
-
-errorToString(error) {
-  switch (error) {
-    case ERROR_A:
-      return "ERROR_A";
-    case ERROR_B = 1: //# 01: compile-time error
-    case ERROR_B: //# none: ok
-      return "ERROR_B";
-    default:
-      return "Unknown error";
-  }
-}
-
-main() {
-  Expect.equals(errorToString(ERROR_A), "ERROR_A");
-  Expect.equals(errorToString(ERROR_B), "ERROR_B");
-  Expect.equals(errorToString(55), "Unknown error");
-}