Add defaultCompletionSorter allowing sorting to be overridden - fixes #24339

pkg/analysis_server/test/mock_sdk.dart

 // Copyright (c) 2014, 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 testing.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/context.dart' as newContext;
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 79 matching lines @@
 
 abstract class List<E> implements Iterable<E> {
   void add(E value);
   E operator [](int index);
   void operator []=(int index, E value);
   Iterator<E> get iterator => null;
   void clear();
 }
 
 abstract class Map<K, V> extends Object {
+  bool containsKey(Object key);
   Iterable<K> get keys;
 }
 
 external bool identical(Object a, Object b);
 
 void print(Object object) {}
 
 class Uri {
   static List<int> parseIPv6Address(String host, [int start = 0, int end]) {
     int parseHex(int start, int end) {
@@ skipping 231 matching lines @@
   bool get isInternal => shortName.startsWith('dart:_');
 
   @override
   bool get isShared => throw unimplemented;
 
   @override
   bool get isVmLibrary => throw unimplemented;
 
   UnimplementedError get unimplemented => new UnimplementedError();
 }