Fix some warnings in the core library identified by the static analyzer.

corelib/src/string_buffer.dart

 // Copyright (c) 2011, 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.
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
  */
 interface StringBuffer default StringBufferImpl {
@@ skipping 19 matching lines @@
 
   /**
    * Adds the string representation of [charCode] to the buffer.
    * Returns [this].
    */
   StringBuffer addCharCode(int charCode);
 
   /**
    * Adds all items in [objects] to the buffer. Returns [:this:].
    */
-  StringBuffer addAll(Collection<Object> objects);
+  StringBuffer addAll(Collection objects);
 
   /**
    * Clears the string buffer. Returns [:this:].
    */
   StringBuffer clear();
 
   /**
    * Returns the contents of buffer as a concatenated string.
    */
   String toString();
 }