Handle invalid deferred prefix declarations.

pkg/compiler/lib/src/compiler.dart

 // Copyright (c) 2012, 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 dart2js.compiler_base;
 
 import 'dart:async' show EventSink, Future;
 
 import '../compiler_new.dart' as api;
 import 'cache_strategy.dart' show CacheStrategy;
@@ skipping 904 matching lines @@
         checkQueues();
       });
 
   void fullyEnqueueLibrary(LibraryElement library, Enqueuer world) {
     void enqueueAll(Element element) {
       fullyEnqueueTopLevelElement(element, world);
     }
     library.implementation.forEachLocalMember(enqueueAll);
     library.imports.forEach((ImportElement import) {
       if (import.isDeferred) {
-        world.addToWorkList(import.prefix.loadLibrary);
+        // `import.prefix` and `loadLibrary` may be `null` when the deferred
+        // import has compile-time errors.
+        GetterElement loadLibrary = import.prefix?.loadLibrary;
+        if (loadLibrary != null) {
+          world.addToWorkList(loadLibrary);
+        }
       }
     });
   }
 
   void fullyEnqueueTopLevelElement(Element element, Enqueuer world) {
     if (element.isClass) {
       ClassElement cls = element;
       cls.ensureResolved(resolution);
       cls.forEachLocalMember(enqueuer.resolution.addToWorkList);
       backend.registerInstantiatedType(cls.rawType, world, globalDependencies);
@@ skipping 1182 matching lines @@
   _ElementScanner(this.scanner);
   void scanLibrary(LibraryElement library) => scanner.scanLibrary(library);
   void scanUnit(CompilationUnitElement unit) => scanner.scan(unit);
 }
 
 class _EmptyEnvironment implements Environment {
   const _EmptyEnvironment();
 
   String valueOf(String key) => null;
 }