Migrate language block 46 - canonical_const ... char_escape.

tests/language_2/cascade_nested_test.dart

 // Copyright (c) 2016, 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 Foo {
   int x;
 }
 
 class Bar {
   Foo foo;
   int y;
 }
 
 main() {
   var bar = new Bar()
     ..foo = (new Foo()..x = 42)
     ..y = 38;
   Expect.isTrue(bar is Bar);
   Expect.isTrue(bar.foo is Foo);
   Expect.equals(bar.foo.x, 42);
   Expect.equals(bar.y, 38);
 }