Fix NPE in ErrorReporter (issue 22980)

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

Index: pkg/analyzer/lib/src/generated/error.dart
diff --git a/pkg/analyzer/lib/src/generated/error.dart b/pkg/analyzer/lib/src/generated/error.dart
index 6ba42dced43df8486232464928071580cc8dc523..9ff7fa198c1c0e91302541dd4adca964eb101b9c 100644
--- a/pkg/analyzer/lib/src/generated/error.dart
+++ b/pkg/analyzer/lib/src/generated/error.dart
@@ -2474,8 +2474,16 @@ class ErrorReporter {
    */
   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);
   }
 
   /**