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

lib/text/ast_to_text.dart

Index: lib/text/ast_to_text.dart
diff --git a/lib/text/ast_to_text.dart b/lib/text/ast_to_text.dart
index b6833bcc266913617e5eab2b08b48465209c1e67..d92b505ad004524e834c98a2838d6cf9803e1186 100644
--- a/lib/text/ast_to_text.dart
+++ b/lib/text/ast_to_text.dart
@@ -839,6 +839,8 @@ class Printer extends Visitor<Null> {
       if (character != null) {
         buffer ??= new StringBuffer(string.substring(0, i));
         buffer.write(character);
+      } else {
+        buffer?.write(string[i]);
       }
     }
     return buffer == null ? string : buffer.toString();