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

lib/text/ast_to_text.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.
 library kernel.ast_to_text;
 
 import '../ast.dart';
 import '../import_table.dart';
 import '../type_propagation/type_propagation.dart';
 
 class Namer<T> {
@@ skipping 821 matching lines @@
     }
   }
 
   String escapeString(String string) {
     StringBuffer buffer;
     for (int i = 0; i < string.length; ++i) {
       String character = getEscapedCharacter(string.codeUnitAt(i));
       if (character != null) {
         buffer ??= new StringBuffer(string.substring(0, i));
         buffer.write(character);
+      } else {
+        buffer?.write(string[i]);
       }
     }
     return buffer == null ? string : buffer.toString();
   }
 
   visitStringConcatenation(StringConcatenation node) {
     if (state == WORD) {
       writeSpace();
     }
     write('"');
@@ skipping 670 matching lines @@
   }
   throw 'illegal ProcedureKind: $kind';
 }
 
 class ExpressionPrinter {
   final Printer writeer;
   final int minimumPrecedence;
 
   ExpressionPrinter(this.writeer, this.minimumPrecedence);
 }