Use APIs from the latest collection package.

lib/src/runner/configuration/load.dart

 // Copyright (c) 2016, 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';
 import 'package:glob/glob.dart';
 import 'package:path/path.dart' as p;
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
 
 import '../../backend/operating_system.dart';
 import '../../backend/platform_selector.dart';
 import '../../backend/test_platform.dart';
 import '../../frontend/timeout.dart';
 import '../../utils.dart';
@@ skipping 376 matching lines @@
         _document.nodes.keys.firstWhere((key) => key.value == field).span,
         _source);
   }
 
   /// Throws a [SourceSpanFormatException] with [message] about [field].
   void _error(String message, String field) {
     throw new SourceSpanFormatException(
         message, _document.nodes[field].span, _source);
   }
 }