Fix recomputing LibraryElement.exportNamespace.

pkg/analyzer/lib/src/task/dart.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 analyzer.src.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 1366 matching lines @@
 
   @override
   TaskDescriptor get descriptor => DESCRIPTOR;
 
   @override
   void internalPerform() {
     LibraryElementImpl library = getRequiredInput(LIBRARY_INPUT);
     //
     // Compute export namespace.
     //
+    library.exportNamespace = null;
     NamespaceBuilder builder = new NamespaceBuilder();
     Namespace namespace = builder.createExportNamespaceForLibrary(library);
     library.exportNamespace = namespace;
     //
     // Update entry point.
     //
     if (library.entryPoint == null) {
       Iterable<Element> exportedElements = namespace.definedNames.values;
       library.entryPoint = exportedElements.firstWhere(
           (element) => element is FunctionElement && element.isEntryPoint,
@@ skipping 4830 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }