Introduce code to generate UnlinkedPublicNamespace directly from an AST.

pkg/analyzer/lib/src/summary/builder.dart

-// Copyright (c) 2014, 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.
-
-/**
- * Format-agnostic infrastructure for building summary objects.
- */
-library analyzer.src.summary.builder;
-
-/**
- * Instances of this class encapsulate the necessary state to keep track of a
- * serialized summary that is in the process of being built.
- *
- * This class is intended to be passed to the constructors of the summary
- * Builder classes.
- */
-class BuilderContext {
-  // Note: at the moment this is a placeholder class since the current
-  // serialization format (JSON) doesn't require any state tracking.  If/when
-  // we switch to a serialization format that requires state tracking, the
-  // state will be stored here.
-}