Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/tree/prettyprint.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.
 
 part of tree;
 
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
@@ skipping 36 matching lines @@
 
   /**
    * Adds given node type to result string.
    * The method "opens" the node, meaning that all output after calling
    * this method and before calling closeNode() will represent contents
    * of given node.
    */
   void openNode(Node node, String type, [Map params]) {
     if (params == null) params = new Map();
     addCurrentIndent();
-    sb.add("<");
+    sb.write("<");
     addBeginAndEndTokensToParams(node, params);
     addTypeWithParams(type, params);
-    sb.add(">\n");
+    sb.write(">\n");
     pushTag(type);
   }
 
   /**
    * Adds given node to result string.
    */
   void openAndCloseNode(Node node, String type, [Map params]) {
     if (params == null) params = new Map();
     addCurrentIndent();
-    sb.add("<");
+    sb.write("<");
     addBeginAndEndTokensToParams(node, params);
     addTypeWithParams(type, params);
-    sb.add("/>\n");
+    sb.write("/>\n");
   }
 
   /**
    * Closes current node type.
    */
   void closeNode() {
     String tag = popTag();
     addCurrentIndent();
-    sb.add("</");
+    sb.write("</");
     addTypeWithParams(tag);
-    sb.add(">\n");
+    sb.write(">\n");
   }
 
   void addTypeWithParams(String type, [Map params]) {
     if (params == null) params = new Map();
-    sb.add("${type}");
+    sb.write("${type}");
     params.forEach((k, v) {
       String value;
       if (v != null) {
         value = v
             .replaceAll("<", "&lt;")
             .replaceAll(">", "&gt;")
             .replaceAll('"', "'");
       } else {
         value = "[null]";
       }
-      sb.add(' $k="$value"');
+      sb.write(' $k="$value"');
     });
   }
 
   void addCurrentIndent() {
-    tagStack.forEach((_) { sb.add(INDENT); });
+    tagStack.forEach((_) { sb.write(INDENT); });
   }
 
   /**
    * Pretty-prints given node tree into string.
    */
   static String prettyPrint(Node node) {
     var p = new PrettyPrinter();
     node.accept(p);
     return p.sb.toString();
   }
@@ skipping 195 matching lines @@
     closeNode();
   }
 
   visitScriptTag(ScriptTag node) {
     visitNodeWithChildren(node, "ScriptTag");
   }
 
   visitChildNode(Node node, String fieldName) {
     if (node == null) return;
     addCurrentIndent();
-    sb.add("<$fieldName>\n");
+    sb.write("<$fieldName>\n");
     pushTag(fieldName);
     node.accept(this);
     popTag();
     addCurrentIndent();
-    sb.add("</$fieldName>\n");
+    sb.write("</$fieldName>\n");
   }
 
   openSendNodeWithFields(Send node, String type) {
     openNode(node, type, {
         "isPrefix" : "${node.isPrefix}",
         "isPostfix" : "${node.isPostfix}",
         "isIndex" : "${node.isIndex}"
     });
     visitChildNode(node.receiver, "receiver");
     visitChildNode(node.selector, "selector");
@@ skipping 134 matching lines @@
   }
 
   visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }