Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/dart_backend/renamer.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;
 
 Function get _compareNodes =>
     compareBy((n) => n.getBeginToken().charOffset);
 
 typedef String _Renamer(Renamable renamable);
@@ skipping 71 matching lines @@
       f(element, map[element]);
     }
   }
 
   String renameType(DartType type, Function renameElement) {
     // TODO(smok): Do not rename type if it is in platform library or
     // js-helpers.
     StringBuffer result = new StringBuffer(renameElement(type.element));
     if (type is InterfaceType) {
       if (!type.isRaw) {
-        result.add('<');
+        result.write('<');
         Link<DartType> argumentsLink = type.typeArguments;
-        result.add(renameType(argumentsLink.head, renameElement));
+        result.write(renameType(argumentsLink.head, renameElement));
         for (Link<DartType> link = argumentsLink.tail; !link.isEmpty;
              link = link.tail) {
-          result.add(',');
-          result.add(renameType(link.head, renameElement));
+          result.write(',');
+          result.write(renameType(link.head, renameElement));
         }
-        result.add('>');
+        result.write('>');
       }
     }
     return result.toString();
   }
 
   String renameConstructor(Element element, ConstructorPlaceholder placeholder,
       Function renameString, Function renameElement) {
     assert(element.isConstructor());
     StringBuffer result = new StringBuffer();
     String name = element.name.slowToString();
     if (element.name != element.getEnclosingClass().name) {
       // Named constructor or factory. Is there a more reliable way to check
       // this case?
       if (!placeholder.isRedirectingCall) {
-        result.add(renameType(placeholder.type, renameElement));
-        result.add('.');
+        result.write(renameType(placeholder.type, renameElement));
+        result.write('.');
       }
       String prefix = '${element.getEnclosingClass().name.slowToString()}\$';
       if (!name.startsWith(prefix)) {
         // Factory for another interface (that is going away soon).
         compiler.internalErrorOnElement(element,
             "Factory constructors for external interfaces are not supported.");
       }
       name = name.substring(prefix.length);
       if (!element.getLibrary().isPlatformLibrary) {
         name = renameString(element.getLibrary(), name);
       }
-      result.add(name);
+      result.write(name);
     } else {
       assert(!placeholder.isRedirectingCall);
-      result.add(renameType(placeholder.type, renameElement));
+      result.write(renameType(placeholder.type, renameElement));
     }
     return result.toString();
   }
 
   Function makeElementRenamer(rename, generateUniqueName) => (element) {
     assert(Elements.isErroneousElement(element) ||
            Elements.isStaticOrTopLevel(element) ||
            element is TypeVariableElement);
     // TODO(smok): We may want to reuse class static field and method names.
     String originalName = element.name.slowToString();
@@ skipping 197 matching lines @@
    * Generates next mini ID with current index and alphabet.
    * Advances current index.
    * In other words, it converts index to visual representation
    * as if digits are given characters.
    */
   String getNextId() {
     // It's like converting index in decimal to [chars] radix.
     int index = nextIdIndex++;
     StringBuffer resultBuilder = new StringBuffer();
     if (index < firstCharAlphabet.length) return firstCharAlphabet[index];
-    resultBuilder.add(firstCharAlphabet[index % firstCharAlphabet.length]);
+    resultBuilder.write(firstCharAlphabet[index % firstCharAlphabet.length]);
     index ~/= firstCharAlphabet.length;
     int length = otherCharsAlphabet.length;
     while (index >= length) {
-      resultBuilder.add(otherCharsAlphabet[index % length]);
+      resultBuilder.write(otherCharsAlphabet[index % length]);
       index ~/= length;
     }
-    resultBuilder.add(otherCharsAlphabet[index]);
+    resultBuilder.write(otherCharsAlphabet[index]);
     return resultBuilder.toString();
   }
 }