Don't update offsets for elements of 'part' and 'part of' directives.

pkg/analyzer/test/src/task/incremental_element_builder_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.incremental_element_builder_test;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/task/incremental_element_builder.dart';
@@ skipping 50 matching lines @@
     {
       Directive newNode = newDirectives[2];
       expect(newNode, same(oldDirectives[1]));
       expect(getNodeText(newNode), "import 'dart:math';");
       ImportElement element = newNode.element;
       expect(element, isNotNull);
       expect(element.nameOffset, newCode.indexOf("import 'dart:math';"));
     }
   }
 
+  test_directives_keepOffset_partOf() {
+    String libCode = '''
+// comment to shift tokens
+library my_lib;
+part 'test.dart';
+''';
+    Source libSource = newSource('/lib.dart', libCode);
+    _buildOldUnit(r'''
+part of my_lib;
+class A {}
+''', libSource);
+    _buildNewUnit(r'''
+part of my_lib;
+class A {}
+''');
+    var oldDirectives = oldUnit.directives;
+    var newDirectives = newUnit.directives;
+    {
+      Directive newNode = newDirectives[0];
+      expect(newNode, same(oldDirectives[0]));
+      expect(getNodeText(newNode), 'part of my_lib;');
+      LibraryElement element = newNode.element;
+      expect(element, isNotNull);
+      expect(element.nameOffset, libCode.indexOf('my_lib;'));
+    }
+  }
+
   test_directives_remove() {
     _buildOldUnit(r'''
 library test;
 import 'dart:async';
 import 'dart:math';
 ''');
     _buildNewUnit(r'''
 library test;
 import 'dart:math';
 ''');
@@ skipping 469 matching lines @@
   }
 
   void _buildNewUnit(String newCode) {
     this.newCode = newCode;
     context.setContents(source, newCode);
     newUnit = context.parseCompilationUnit(source);
     new IncrementalCompilationUnitElementBuilder(oldUnit, newUnit).build();
     expect(newUnit.element, unitElement);
   }
 
-  void _buildOldUnit(String oldCode) {
+  void _buildOldUnit(String oldCode, [Source libSource]) {
     this.oldCode = oldCode;
     source = newSource('/test.dart', oldCode);
-    oldUnit = context.resolveCompilationUnit2(source, source);
+    if (libSource == null) {
+      libSource = source;
+    }
+    oldUnit = context.resolveCompilationUnit2(source, libSource);
     unitElement = oldUnit.element;
     expect(unitElement, isNotNull);
   }
 }