Remove Collection, Collections and clean up List/Set/Queue implementations of retain/remove.

pkg/args/lib/args.dart

 // Copyright (c) 2013, 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.
 
 /**
  * This library lets you define parsers for parsing raw command-line arguments
  * into a set of options and values using [GNU][] and [POSIX][] style options.
  *
  * ## Defining options ##
  *
@@ skipping 399 matching lines @@
   /** Gets the parsed command-line option named [name]. */
   operator [](String name) {
     if (!_options.containsKey(name)) {
       throw new ArgumentError(
           'Could not find an option named "$name".');
     }
 
     return _options[name];
   }
 
-  /** Get the names of the options as a [Collection]. */
-  Collection<String> get options => _options.keys.toList(growable: false);
+  /** Get the names of the options as an [Iterable]. */
+  Iterable<String> get options => _options.keys;
 }