Don't recurse into (external) compilation units.

pkg/analyzer/lib/src/task/incremental_element_builder.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.incremental_element_builder;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 255 matching lines @@
 }
 
 /**
  * Updates name offsets of [Element]s according to the [map].
  */
 class _UpdateElementOffsetsVisitor extends GeneralizingElementVisitor {
   final Map<int, int> map;
 
   _UpdateElementOffsetsVisitor(this.map);
 
-  visitElement(Element element) {
+  void visitElement(Element element) {
+    if (element is CompilationUnitElement) {
+      return;
+    }
+    if (element.isSynthetic) {
+      return;
+    }
     int oldOffset = element.nameOffset;
-    if (oldOffset != -1) {
-      int newOffset = map[oldOffset];
-      assert(newOffset != null);
-      (element as ElementImpl).nameOffset = newOffset;
-    }
+    int newOffset = map[oldOffset];
+    assert(newOffset != null);
+    (element as ElementImpl).nameOffset = newOffset;
     if (element is! LibraryElement) {
       super.visitElement(element);
     }
   }
 }