We need a CompilationUnit with element.library set.

pkg/analyzer/test/src/context/context_test.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 analyzer.test.src.context.context_test;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 4606 matching lines @@
     context.setContents(
         a,
         r'''
 class A {}
 class B2 {}
 ''');
     _assertValidAllLibraryUnitResults(b);
     _assertValid(b, LIBRARY_ERRORS_READY);
   }
 
+  void test_sequence_useAnyResolvedUnit_needsLibraryElement() {
+    Source a = addSource(
+        '/a.dart',
+        r'''
+class A {}
+class B {}
+''');
+    // Perform analysis until we get RESOLVED_UNIT1.
+    // But it does not have 'library' set, so `unitElement.context` is `null`.
+    LibrarySpecificUnit aUnitTarget = new LibrarySpecificUnit(a, a);
+    while (context.getResult(aUnitTarget, RESOLVED_UNIT1) == null) {
+      context.performAnalysisTask();
+    }
+    // There was a bug with exception in incremental element builder.
+    // We should not attempt to use `unitElement.context`.
+    // It calls `unitElement.library`, which might be not set yet.
+    context.setContents(
+        a,
+        r'''
+class A {}
+class B2 {}
+''');
+    // OK, no exceptions.
+  }
+
   void test_unusedName_class_add() {
     Source a = addSource(
         '/a.dart',
         r'''
 class A {}
 class B {}
 class C {}
 ''');
     Source b = addSource(
         '/b.dart',
@@ skipping 588 matching lines @@
    * Initialize the visitor.
    */
   _ElementGatherer();
 
   @override
   void visitElement(Element element) {
     elements[element] = element;
     super.visitElement(element);
   }
 }