Implement 'topLevelLibraryDeclarations' getter to return all libraries with exported top-level decl…

pkg/analyzer/lib/src/dart/analysis/top_level_declaration.dart

 // Copyright (c) 2016, 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/source.dart';
 
 /**
  * Information about a single top-level declaration.
  */
 class TopLevelDeclaration {
@@ skipping 15 matching lines @@
  */
 class TopLevelLibraryDeclarations {
   /**
    * The source of the library.
    */
   final Source source;
 
   /**
    * Top-level declarations in the export namespace of the library.
    */
-  final List<TopLevelDeclaration> declarations = [];
+  final List<TopLevelDeclaration> declarations;
 
-  TopLevelLibraryDeclarations(this.source);
+  TopLevelLibraryDeclarations(this.source, this.declarations);
 }