Introduce code to generate UnlinkedPublicNamespace directly from an AST.

pkg/analyzer/tool/summary/build_sdk_summary.dart

 import 'dart:io';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_io.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/sdk_io.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/summarize_elements.dart';
 
 main(List<String> args) {
   if (args.length < 1 || args.length > 2) {
     _printUsage();
     return;
   }
   //
   // Prepare output file path.
@@ skipping 60 matching lines @@
  * The name of the SDK summary builder application.
  */
 const BINARY_NAME = "build_sdk_summary";
 
 /**
  * Print information about how to use the SDK summary builder.
  */
 void _printUsage() {
   print('Usage: $BINARY_NAME output_file_path [sdk_path]');
 }