Migrate language block 46 - canonical_const ... char_escape.

tests/language_2/catch_liveness_test.dart

 // Copyright (c) 2015, 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";
 
 @AssumeDynamic()
 @NoInline()
 foo() => 1;
 
 @AssumeDynamic()
 @NoInline()
 throwException() => throw 'x';
 
 main() {
   var x = 10;
   var e2 = null;
   try {
     var t = foo();
     throwException();
     print(t);
     x = 3;
   } catch (e) {
     Expect.equals(10, x);
     e2 = e;
   }
   Expect.equals(10, x);
   Expect.equals('x', e2);
 }