Use APIs from the latest collection package.

lib/src/runner/configuration.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:io';
 
 import 'package:boolean_selector/boolean_selector.dart';
-import 'package:collection/collection.dart' hide mapMap, mergeMaps;
+import 'package:collection/collection.dart';
 import 'package:glob/glob.dart';
 import 'package:path/path.dart' as p;
 
 import '../backend/metadata.dart';
 import '../backend/platform_selector.dart';
 import '../backend/test_platform.dart';
 import '../frontend/timeout.dart';
 import '../util/io.dart';
-import '../utils.dart';
 import 'configuration/args.dart' as args;
 import 'configuration/load.dart';
 import 'configuration/values.dart';
 
 /// A class that encapsulates the command-line configuration of the test runner.
 class Configuration {
   /// An empty configuration with only default values.
   ///
   /// Using this is slightly more efficient than manually constructing a new
   /// configuration with no arguments.
@@ skipping 520 matching lines @@
 
   /// Merges two maps whose values are [Configuration]s.
   ///
   /// Any overlapping keys in the maps have their configurations merged in the
   /// returned map.
   Map<Object, Configuration> _mergeConfigMaps(Map<Object, Configuration> map1,
           Map<Object, Configuration> map2) =>
       mergeMaps(map1, map2,
           value: (config1, config2) => config1.merge(config2));
 }