Fix prelinker when a local name shadows an import prefix.

pkg/analyzer/lib/src/summary/prelink.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:analyzer/src/generated/utilities_dart.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/idl.dart';
 import 'package:analyzer/src/summary/name_filter.dart';
 
 /**
@@ skipping 333 matching lines @@
 
   /**
    * Filter the export namespace for the library whose URI is reachable from
    * [definingUnit] via [relativeUri], retaining only those names accepted by
    * [combinators], and store the resulting names in [result].  Names that
    * already exist in [result] are not overwritten.
    */
   void filterExportNamespace(String relativeUri,
       List<UnlinkedCombinator> combinators, Map<String, _Meaning> result) {
     Map<String, _Meaning> exportNamespace = computeExportNamespace(relativeUri);
+    if (result == null) {
+      // This can happen if the import prefix was shadowed by a local name, so
+      // the imported symbols are inaccessible.
+      return;
+    }
     NameFilter filter = new NameFilter.forUnlinkedCombinators(combinators);
     exportNamespace.forEach((String name, _Meaning meaning) {
       if (filter.accepts(name) && !result.containsKey(name)) {
         result[name] = meaning;
       }
     });
   }
 
   /**
    * Wrapper around [getImport] that caches the return value in [importCache].
@@ skipping 171 matching lines @@
   String _selectUri(
       String defaultUri, List<UnlinkedConfiguration> configurations) {
     for (UnlinkedConfiguration configuration in configurations) {
       if (getDeclaredVariable(configuration.name) == configuration.value) {
         return configuration.uri;
       }
     }
     return defaultUri;
   }
 }