Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/universe/universe.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 universe;
 
 import '../closure.dart';
 import '../elements/elements.dart';
 import '../dart2jslib.dart';
 import '../dart_types.dart';
@@ skipping 397 matching lines @@
     orderedNamedArguments.sort((SourceString first, SourceString second) {
       return first.slowToString().compareTo(second.slowToString());
     });
     return orderedNamedArguments;
   }
 
   String namedArgumentsToString() {
     if (namedArgumentCount > 0) {
       StringBuffer result = new StringBuffer();
       for (int i = 0; i < namedArgumentCount; i++) {
-        if (i != 0) result.add(', ');
-        result.add(namedArguments[i].slowToString());
+        if (i != 0) result.write(', ');
+        result.write(namedArguments[i].slowToString());
       }
       return "[$result]";
     }
     return '';
   }
 
   String toString() {
     String named = '';
     String type = '';
     if (namedArgumentCount > 0) named = ', named=${namedArgumentsToString()}';
@@ skipping 101 matching lines @@
       if (cls.isSubclassOf(other)) {
         // Resolve an invocation of [element.name] on [self]. If it
         // is found, this selector is a candidate.
         return hasElementIn(self, element) && appliesUntyped(element, compiler);
       }
     }
 
     return false;
   }
 }