First partial step at removing interfaces in libraries.

lib/core/collection.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 [Collection] interface is the public interface of all
  * collections.
  */
-interface Collection<E> extends Iterable<E> {
+abstract class Collection<E> extends Iterable<E> {
   /**
    * Applies the function [f] to each element of this collection.
    */
   void forEach(void f(E element));
 
   /**
    * Returns a new collection with the elements [: f(e) :]
    * for each element [:e:] of this collection.
    *
    * Note on typing: the return type of f() could be an arbitrary
@@ skipping 39 matching lines @@
   /**
    * Returns true if there is no element in this collection.
    */
   bool isEmpty();
 
   /**
    * Returns the number of elements in this collection.
    */
   int get length;
 }