Implement SourceFactory.forUri2() to fix front_end after f28e8afb23b2.

pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart

Index: pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart
diff --git a/pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart b/pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart
index aa51fbf29b7c4e844ffce2a1ec65233731c0581c..88ac8ecc2d2c534f7a56c3de6e6c08afe6189e50 100644
--- a/pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart
+++ b/pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart
@@ -268,8 +268,13 @@ class _SourceFactoryProxy implements SourceFactory {
   @override
   Source forUri(String absoluteUri) {
     Uri uri = Uri.parse(absoluteUri);
+    return forUri2(uri);
+  }
+
+  @override
+  Source forUri2(Uri absoluteUri) {
     return new _SourceProxy(
-        uri, _fileRepository.pathForUri(uri, allocate: true));
+        absoluteUri, _fileRepository.pathForUri(absoluteUri, allocate: true));
   }
 
   noSuchMethod(Invocation invocation) => unimplemented();