Fix single library test.

pkg/docgen/lib/docgen.dart

 // Copyright (c) 2013, 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.
 
 /// **docgen** is a tool for creating machine readable representations of Dart
 /// code metadata, including: classes, members, comments and annotations.
 ///
 /// docgen is run on a `.dart` file or a directory containing `.dart` files.
 ///
 ///      $ dart docgen.dart [OPTIONS] [FILE/DIR]
@@ skipping 525 matching lines @@
 
   var classScope = currentClass == null ?
       null : lookupFunc(currentClass, name);
   if (classScope != null) return docName(classScope);
 
   var libraryScope = currentLibrary == null ?
       null : lookupFunc(currentLibrary, name);
   if (libraryScope != null) return docName(libraryScope);
 
   // Look in the dart core library scope.
-  var coreScope = lookupFunc(_coreLibrary, name);
+  var coreScope = _coreLibrary == null? null : lookupFunc(_coreLibrary, name);
   if (coreScope != null) return docName(_coreLibrary);
 
   // If it's a reference that starts with a another library name, then it
   // looks for a match of that library name in the other sdk libraries.
   if(name.contains('.')) {
     var index = name.indexOf('.');
     var libraryName = name.substring(0, index);
     var remainingName = name.substring(index + 1);
     foundLibraryName(library) => library.uri.pathSegments[0] == libraryName;
 
@@ skipping 810 matching lines @@
 /// Remove statics from the map of inherited items before adding them.
 Map _filterStatics(Map items) {
   var result = {};
   items.forEach((name, item) {
     if (!item.isStatic) {
       result[name] = item;
     }
   });
   return result;
 }