Fix equality between subclasses of Source

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 analyzer.file_system.memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core' hide Resource;
 
@@ skipping 431 matching lines @@
     } else if (scheme == DartUriResolver.DART_SCHEME) {
       return UriKind.DART_URI;
     } else if (scheme == FileUriResolver.FILE_SCHEME) {
       return UriKind.FILE_URI;
     }
     return UriKind.FILE_URI;
   }
 
   @override
   bool operator ==(other) {
-    return other is _MemoryFileSource && other.id == id;
+    if (other is _MemoryFileSource) {
+      return id == other.id;
+    } else if (other is Source) {
+      return uri == other.uri;
+    }
+    return false;
   }
 
   @override
   bool exists() => file.exists;
 
   @override
   String toString() => file.toString();
 }
 
 /**
@@ skipping 107 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }