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

pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/dart/analysis/byte_store.dart';
@@ skipping 250 matching lines @@
   AnalysisContext context;
 
   _SourceFactoryProxy(this.dartSdk, this._fileRepository);
 
   @override
   SourceFactory clone() => new _SourceFactoryProxy(dartSdk, _fileRepository);
 
   @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();
 
   Source resolveUri(Source containingSource, String containedUri) {
     // TODO(paulberry): re-use code from dependency_grapher_impl, and support
     // SDK URI resolution logic.
     String absoluteUri =
         resolveRelativeUri(containingSource?.uri, Uri.parse(containedUri))
             .toString();
     return forUri(absoluteUri);
   }
 
   @override
   Uri restoreUri(Source source) => source.uri;
 }
 
 class _SourceProxy extends BasicSource {
   @override
   final String fullName;
 
   _SourceProxy(Uri uri, this.fullName) : super(uri);
 
   int get modificationStamp => 0;
 
   noSuchMethod(Invocation invocation) => unimplemented();
 }