Clean up hacky handling of "dart:" URIs in incremental resolved AST generator.

pkg/front_end/test/src/base/file_repository_test.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 'package:front_end/src/base/file_repository.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
@@ skipping 26 matching lines @@
 
   test_pathForUri() {
     var uri1 = Uri.parse('file:///foo/bar.dart');
     var path1 = fileRepository.store(uri1, 'contents1');
     var uri2 = Uri.parse('package:foo/bar.dart');
     var path2 = fileRepository.store(uri2, 'contents2');
     expect(fileRepository.pathForUri(uri1), path1);
     expect(fileRepository.pathForUri(uri2), path2);
   }
 
+  test_pathForUri_allocate() {
+    var uri1 = Uri.parse('file:///foo/bar.dart');
+    var path1 = fileRepository.pathForUri(uri1, allocate: true);
+    var uri2 = Uri.parse('package:foo/bar.dart');
+    var path2 = fileRepository.pathForUri(uri2, allocate: true);
+    expect(fileRepository.store(uri1, 'contents1'), path1);
+    expect(fileRepository.store(uri2, 'contents2'), path2);
+  }
+
   test_store() {
     var uri = Uri.parse('file:///foo/bar.dart');
     var path = fileRepository.store(uri, 'contents1');
     expect(path, endsWith('.dart'));
     expect(fileRepository.contentsForPath(path), 'contents1');
     expect(fileRepository.getContentsForTesting(), {path: 'contents1'});
     expect(fileRepository.store(uri, 'contents2'), path);
     expect(fileRepository.contentsForPath(path), 'contents2');
     expect(fileRepository.getContentsForTesting(), {path: 'contents2'});
   }
 
   test_uriForPath() {
     var uri1 = Uri.parse('file:///foo/bar.dart');
     var path1 = fileRepository.store(uri1, 'contents1');
     var uri2 = Uri.parse('package:foo/bar.dart');
     var path2 = fileRepository.store(uri2, 'contents2');
     expect(fileRepository.uriForPath(path1), uri1);
     expect(fileRepository.uriForPath(path2), uri2);
   }
 }