Fix a bug in printing of string literals with escape sequences.

testcases/input/stringliteral.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.
+var color = 'brown';
+var thing = 'lazy dog';
+var phrase = "The quick $color fox\njumped over the $thing.\n";
+var adjacent = '$color$color$color';
+var linebreaks = '$color\n$color\n$color';
+var other = '$color\n is \n$color';
+
+main() {}