Remove deprecated StringBuffer.add, addAll and addCharCode.

tools/line_doc_comments.dart

 #!/usr/bin/env dart
 
 /// Converts block-style Doc comments in Dart code to line style.
 library line_doc_comments;
 import 'dart:io';
 
 import '../pkg/pathos/lib/path.dart' as path;
 
 final oneLineBlock = new RegExp(r'^(\s*)/\*\*\s?(.*)\*/\s*$');
 final startBlock = new RegExp(r'^(\s*)/\*\*(.*)$');
@@ skipping 65 matching lines @@
             // Got comment on /** line.
             line = match[2];
           } else {
             // Just a pointless line, delete it!
             line = null;
           }
         }
       }
     }
 
-    if (line != null) buffer.add('$line\n');
+    if (line != null) buffer.write('$line\n');
   }
 
   return buffer.toString();
 }