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

pkg/scheduled_test/lib/src/schedule.dart

Index: pkg/scheduled_test/lib/src/schedule.dart
diff --git a/pkg/scheduled_test/lib/src/schedule.dart b/pkg/scheduled_test/lib/src/schedule.dart
index be49383f1a2fe0578000e86b383fd0e9f4c6ec0e..2569b4bd9efebd4b70f2e7bf4a4cfc4035c051d3 100644
--- a/pkg/scheduled_test/lib/src/schedule.dart
+++ b/pkg/scheduled_test/lib/src/schedule.dart
@@ -348,7 +348,7 @@ class ScheduleState {
 class TaskQueue {
   // TODO(nweiz): make this a read-only view when issue 8321 is fixed.
   /// The tasks in the queue.
-  Collection<Task> get contents => _contents;
+  Iterable<Task> get contents => _contents;
   final _contents = new Queue<Task>();
 
   /// The name of the queue, for debugging purposes.
@@ -380,7 +380,7 @@ class TaskQueue {
   // TODO(nweiz): make this a read-only view when issue 8321 is fixed.
   /// The descriptions of all callbacks that are blocking the completion of
   /// [this].
-  Collection<String> get pendingCallbacks => _pendingCallbacks;
+  Iterable<String> get pendingCallbacks => _pendingCallbacks;
   final _pendingCallbacks = new Queue<String>();
 
   /// A completer that will be completed once [_pendingCallbacks] becomes empty