Migrate language block 46 - canonical_const ... char_escape.

tests/language_strong/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);
-}