Fix NPE in ErrorReporter (issue 22980)

pkg/analyzer/lib/src/generated/error.dart

 // Copyright (c) 2014, 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 engine.error;
 
 import 'dart:collection';
 
 import 'ast.dart' show AstNode;
 import 'element.dart';
@@ skipping 2456 matching lines @@
   void reportError(AnalysisError error) {
     _errorListener.onError(error);
   }
 
   /**
    * Report an error with the given [errorCode] and [arguments]. The [element]
    * is used to compute the location of the error.
    */
   void reportErrorForElement(
       ErrorCode errorCode, Element element, List<Object> arguments) {
-    reportErrorForOffset(
-        errorCode, element.nameOffset, element.displayName.length, arguments);
+    String displayName = element.displayName;
+    int length = 0;
+    if (displayName != null) {
+      length = displayName.length;
+    } else if (element is ImportElement) {
+      length = 6; // 'import'.length
+    } else if (element is ExportElement) {
+      length = 6; // 'export'.length
+    }
+    reportErrorForOffset(errorCode, element.nameOffset, length, arguments);
   }
 
   /**
    * Report an error with the given [errorCode] and [arguments].
    * The [node] is used to compute the location of the error.
    *
    * If the arguments contain the names of two or more types, the method
    * [reportTypeErrorForNode] should be used and the types
    * themselves (rather than their names) should be passed as arguments.
    */
@@ skipping 2385 matching lines @@
    * Initialize a newly created error code to have the given [name].
    */
   const TodoCode(String name) : super(name, "{0}");
 
   @override
   ErrorSeverity get errorSeverity => ErrorSeverity.INFO;
 
   @override
   ErrorType get type => ErrorType.TODO;
 }