fix named constructor completion

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/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk.dart';
@@ skipping 15 matching lines @@
 class Function {}
 class StackTrace {}
 class Symbol {}
 class Type {}
 
 abstract class Comparable<T> {
   int compareTo(T other);
 }
 
 class String implements Comparable<String> {
+  external factory String.fromCharCodes(Iterable<int> charCodes,
+                                        [int start = 0, int end]);
   bool get isEmpty => false;
   bool get isNotEmpty => false;
   int get length => 0;
 }
 
 class bool extends Object {}
 abstract class num implements Comparable<num> {
   bool operator <(num other);
   bool operator <=(num other);
   bool operator >(num other);
@@ skipping 211 matching lines @@
   bool get isInternal => throw unimplemented;
 
   @override
   bool get isShared => throw unimplemented;
 
   @override
   bool get isVmLibrary => throw unimplemented;
 
   UnimplementedError get unimplemented => new UnimplementedError();
 }