Introduce code to generate UnlinkedPublicNamespace directly from an AST.

pkg/analyzer/test/src/summary/resynthesize_test.dart

 // Copyright (c) 2015, 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 test.src.serialization.elements_test;
 
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/source.dart';
-import 'package:analyzer/src/summary/builder.dart';
+import 'package:analyzer/src/summary/base.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/resynthesize.dart';
 import 'package:analyzer/src/summary/summarize_elements.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../generated/resolver_test.dart';
 import '../../reflective_tests.dart';
 
 main() {
   groupSep = ' | ';
@@ skipping 1027 matching lines @@
   }
 
   test_variable_const() {
     checkLibrary('const int i = 0;');
   }
 
   test_variables() {
     checkLibrary('int i; int j;');
   }
 }