Uncomment and test fixes for importing libraries.

pkg/analyzer/lib/file_system/memory_file_system.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 memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
@@ skipping 199 matching lines @@
     return '${new String.fromCharCode(uriKind.encoding)}${_file.path}';
   }
 
   @override
   String get fullName => _file.path;
 
   @override
   int get hashCode => _file.hashCode;
 
   @override
-  bool get isInSystemLibrary => false;
+  bool get isInSystemLibrary => uriKind == UriKind.DART_URI;
 
   @override
   int get modificationStamp => _file._timestamp;
 
   @override
   String get shortName => _file.shortName;
 
   @override
   bool operator ==(other) {
     if (other is _MemoryFileSource) {
@@ skipping 101 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }