Fix NPE in building element model when part URI could not be resolved

pkg/analyzer/test/src/task/dart_test.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 test.src.task.dart_test;
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 907 matching lines @@
 library lib;
 part 'part.dart';
 ''',
       '/part.dart': '''
 // no part of
 '''
     });
     _assertErrorsWithCodes([CompileTimeErrorCode.PART_OF_NON_PART]);
   }
 
+  test_perform_invalidUri_part() {
+    _performBuildTask({
+      '/lib.dart': '''
+library lib;
+part '//////////';
+'''
+    });
+    expect(libraryElement.parts, isEmpty);
+  }
+
   test_perform_isLaunchable_inDefiningUnit() {
     _performBuildTask({
       '/lib.dart': '''
 library lib;
 main() {
 }
 '''
     });
     expect(outputs[IS_LAUNCHABLE], isTrue);
     expect(libraryElement.entryPoint, isNotNull);
@@ skipping 1576 matching lines @@
   /**
    * Fill [errorListener] with [result] errors in the current [task].
    */
   void _fillErrorListener(ResultDescriptor<List<AnalysisError>> result) {
     List<AnalysisError> errors = task.outputs[result];
     expect(errors, isNotNull, reason: result.name);
     errorListener = new GatheringErrorListener();
     errorListener.addAll(errors);
   }
 }