Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/util/link_implementation.dart

 // Copyright (c) 2011, 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.
 
 part of util_implementation;
 
 class LinkIterator<T> implements Iterator<T> {
   T _current;
   Link<T> _link;
 
@@ skipping 18 matching lines @@
 
   LinkEntry(T this.head, [Link<T> tail])
     : this.tail = ((tail == null) ? new Link<T>() : tail);
 
   Link<T> prepend(T element) {
     // TODO(ahe): Use new Link<T>, but this cost 8% performance on VM.
     return new LinkEntry<T>(element, this);
   }
 
   void printOn(StringBuffer buffer, [separatedBy]) {
-    buffer.add(head);
+    buffer.write(head);
     if (separatedBy == null) separatedBy = '';
     for (Link link = tail; !link.isEmpty; link = link.tail) {
-      buffer.add(separatedBy);
-      buffer.add(link.head);
+      buffer.write(separatedBy);
+      buffer.write(link.head);
     }
   }
 
   String toString() {
     StringBuffer buffer = new StringBuffer();
-    buffer.add('[ ');
+    buffer.write('[ ');
     printOn(buffer, ', ');
-    buffer.add(' ]');
+    buffer.write(' ]');
     return buffer.toString();
   }
 
   Link<T> reverse() {
     Link<T> result = const Link();
     for (Link<T> link = this; !link.isEmpty; link = link.tail) {
       result = result.prepend(link.head);
     }
     return result;
   }
@@ skipping 71 matching lines @@
     if (head == null) {
       head = entry;
     } else {
       lastLink.tail = entry;
     }
     lastLink = entry;
   }
 
   bool get isEmpty => length == 0;
 }