Reapply conversion of most core interfaces to abstract classes.

lib/coreimpl/collections.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.
 
 /**
  * The [Collections] class implements static methods useful when
  * writing a class that implements [Collection] and the [iterator]
  * method.
  */
 class Collections {
@@ skipping 120 matching lines @@
       } else {
         _emitCollection(o, result, visiting);
       }
     } else if (o is Map) {
       if (_containsRef(visiting, o)) {
         result.add('{...}');
       } else {
         Maps._emitMap(o, result, visiting);
       }
     } else { // o is neither a collection nor a map
-      result.add(o == null ? 'null' : o);  // TODO(jjb): remove the null check
+      result.add(o);
     }
   }
 
   /**
    * Returns true if the specified collection contains the specified object
    * reference.
    */
   static _containsRef(Collection c, Object ref) {
     for (var e in c) {
       if (e === ref) return true;
     }
     return false;
   }
 }