Migrate language block 46 - canonical_const ... char_escape.

tests/language_2/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");
 }