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

pkg/scheduled_test/lib/src/schedule_error.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.
 
 library schedule_error;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'schedule.dart';
 import 'task.dart';
 import 'utils.dart';
 
 /// A wrapper for errors that occur during a scheduled test.
 class ScheduleError extends AsyncError {
   /// The schedule during which this error occurred.
   final Schedule schedule;
 
   /// The task that was running when this error occurred. This may be `null` if
   /// there was no such task.
   final Task task;
 
   /// The task queue that was running when this error occured. This may be
   /// `null` if there was no such queue.
   final TaskQueue queue;
 
   /// The descriptions of out-of-band callbacks that were pending when this
   /// error occurred.
-  final Collection<String> pendingCallbacks;
+  final Iterable<String> pendingCallbacks;
 
   /// The state of the schedule at the time the error was detected.
   final ScheduleState _stateWhenDetected;
 
   int get hashCode => schedule.hashCode ^ task.hashCode ^ queue.hashCode ^
       _stateWhenDetected.hashCode ^ error.hashCode ^ stackTrace.hashCode ^
       cause.hashCode;
 
   /// Creates a new [ScheduleError] wrapping [error]. The metadata in
   /// [AsyncError]s and [ScheduleError]s will be preserved.
@@ skipping 66 matching lines @@
       result.write("\n\n");
       result.writeln("Pending out-of-band callbacks:");
       for (var callback in pendingCallbacks) {
         result.writeln(prefixLines(callback, firstPrefix: "* "));
       }
     }
 
     return result.toString().trim();
   }
 }