Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/dart_backend/backend.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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 422 matching lines @@
     final sortedTopLevels = sortElements(topLevelElements);
     final sortedClassMembers = new Map<ClassElement, List<Element>>();
     classMembers.forEach((classElement, members) {
       sortedClassMembers[classElement] = sortElements(members);
     });
 
     if (outputAst) {
       // TODO(antonm): Ideally XML should be a separate backend.
       // TODO(antonm): obey renames and minification, at least as an option.
       StringBuffer sb = new StringBuffer();
-      outputElement(element) { sb.add(parse(element).toDebugString()); }
+      outputElement(element) { sb.write(parse(element).toDebugString()); }
 
       // Emit XML for AST instead of the program.
       for (final topLevel in sortedTopLevels) {
         if (topLevel.isClass()) {
           // TODO(antonm): add some class info.
           sortedClassMembers[topLevel].forEach(outputElement);
         } else {
           outputElement(topLevel);
         }
       }
@@ skipping 41 matching lines @@
   log(String message) => compiler.log('[DartBackend] $message');
 }
 
 class EmitterUnparser extends Unparser {
   final Map<Node, String> renames;
 
   EmitterUnparser(this.renames);
 
   visit(Node node) {
     if (node != null && renames.containsKey(node)) {
-      sb.add(renames[node]);
+      sb.write(renames[node]);
     } else {
       super.visit(node);
     }
   }
 
   unparseSendReceiver(Send node, {bool spacesNeeded: false}) {
     // TODO(smok): Remove ugly hack for library prefices.
     if (node.receiver != null && renames[node.receiver] == '') return;
     super.unparseSendReceiver(node, spacesNeeded: spacesNeeded);
   }
 
   unparseFunctionName(Node name) {
     if (name != null && renames.containsKey(name)) {
-      sb.add(renames[name]);
+      sb.write(renames[name]);
     } else {
       super.unparseFunctionName(name);
     }
   }
 }
 
 
 /**
  * Some elements are not recorded by resolver now,
  * for example, typedefs or classes which are only
@@ skipping 53 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().canonicalUri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Iterable<Element> elements) =>
     sorted(elements, compareElements);