Migrate language block 46 - canonical_const ... char_escape.

tests/language_2/cascade_precedence_test.dart

 // 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.
 
 import "package:expect/expect.dart";
 
 class A {
   int value;
   A(this.value);
   void set(int value) {
@@ skipping 163 matching lines @@
   Expect.equals(box.value, b);
 
   // New cascades are allowed inside an expressionWithoutCascade if properly
   // delimited.
   box
     ..x = (a
       ..set(42)
       ..test(42))
     ..x.test(42);
 }