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

pkg/args/lib/args.dart

Index: pkg/args/lib/args.dart
diff --git a/pkg/args/lib/args.dart b/pkg/args/lib/args.dart
index 48cef1642ece730acba2bbc42027c05a910255b0..1337b46a6a8f40ac92b72a14c175eee7e8c867bb 100644
--- a/pkg/args/lib/args.dart
+++ b/pkg/args/lib/args.dart
@@ -417,7 +417,7 @@ class ArgResults {
     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;
 }