Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/source_file.dart

 // Copyright (c) 2012, 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 source_file;
 
 import 'dart:math';
 
 import 'colors.dart' as colors;
 
@@ skipping 58 matching lines @@
    * in the file.
    */
   String getLocationMessage(String message, int start, int end,
                             bool includeText, String color(String x)) {
     var line = getLine(start);
     var column = getColumn(line, start);
 
     var buf = new StringBuffer(
         '${filename}:${line + 1}:${column + 1}: $message');
     if (includeText) {
-      buf.add('\n');
+      buf.write('\n');
       var textLine;
       // +1 for 0-indexing, +1 again to avoid the last line of the file
       if ((line + 2) < _lineStarts.length) {
         textLine = text.substring(_lineStarts[line], _lineStarts[line+1]);
       } else {
         textLine = '${text.substring(_lineStarts[line])}\n';
       }
 
       int toColumn = min(column + (end-start), textLine.length);
-      buf.add(textLine.substring(0, column));
-      buf.add(color(textLine.substring(column, toColumn)));
-      buf.add(textLine.substring(toColumn));
+      buf.write(textLine.substring(0, column));
+      buf.write(color(textLine.substring(column, toColumn)));
+      buf.write(textLine.substring(toColumn));
 
       int i = 0;
       for (; i < column; i++) {
-        buf.add(' ');
+        buf.write(' ');
       }
 
       for (; i < toColumn; i++) {
-        buf.add(color('^'));
+        buf.write(color('^'));
       }
     }
 
     return buf.toString();
   }
 }