Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/code_buffer.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 dart2js;
 
 class CodeBuffer implements StringBuffer {
   StringBuffer buffer;
   List<CodeBufferMarker> markers;
   int lastBufferOffset = 0;
@@ skipping 15 matching lines @@
   }
   /**
    * Converts [object] to a string and adds it to the buffer. If [object] is a
    * [CodeBuffer], adds its markers to [markers].
    */
   CodeBuffer write(var object) {
     if (object is CodeBuffer) {
       return addBuffer(object);
     }
     if (mappedRangeCounter == 0) setSourceLocation(null);
-    buffer.add(object.toString());
+    buffer.write(object);
     return this;
   }
 
   CodeBuffer writeAll(Iterable<Object> objects) {
     for (var object in objects) {
       write(object);
     }
     return this;
   }
 
   CodeBuffer writeln(var object) {
     return write(object).write("\n");
   }
 
   CodeBuffer addBuffer(CodeBuffer other) {
     if (other.markers.length > 0) {
       CodeBufferMarker firstMarker = other.markers[0];
       int offsetDelta =
           buffer.length + firstMarker.offsetDelta - lastBufferOffset;
       markers.add(new CodeBufferMarker(offsetDelta,
                                        firstMarker.sourcePosition));
       for (int i = 1; i < other.markers.length; ++i) {
         markers.add(other.markers[i]);
       }
       lastBufferOffset = buffer.length + other.lastBufferOffset;
     }
-    buffer.add(other.getText());
+    buffer.write(other.getText());
     return this;
   }
 
   CodeBuffer addAll(Iterable<Object> iterable) => writeAll(iterable);
 
   CodeBuffer addCharCode(int charCode) => writeCharCode(charCode);
 
   CodeBuffer writeCharCode(int charCode) {
     return write(new String.fromCharCodes([charCode]));
   }
 
   CodeBuffer clear() {
-    buffer.clear();
+    buffer = new StringBuffer();
     markers.clear();
     lastBufferOffset = 0;
     return this;
   }
 
   String toString() {
     throw "Don't use CodeBuffer.toString() since it drops sourcemap data.";
   }
 
   String getText() {
@@ skipping 23 matching lines @@
     });
   }
 }
 
 class CodeBufferMarker {
   final int offsetDelta;
   final sourcePosition;
 
   CodeBufferMarker(this.offsetDelta, this.sourcePosition);
 }