Add FileSystemEntity.identical, to test if two file system objects are the same.

sdk/lib/_internal/compiler/implementation/lib/io_patch.dart

Index: sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
index 11f095765cd24dd97da58fc1f7dcd13978e84dc7..a0ef33c80f8f79828fe97a2f9185f3d30666d7a0 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
@@ -57,6 +57,9 @@ patch class FileSystemEntity {
   patch static int _getType(String path, bool followLinks) {
     throw new UnsupportedError("FileSystemEntity._getType");
   }
+  patch static bool _identical(String file1, String file2) {
+    throw new UnsupportedError("FileSystemEntity._getType");
+  }
 }
 
 patch class _File {