Ensure that prefix?.loadLibrary() generates the proper compile-time error.

pkg/analyzer/lib/src/generated/element_resolver.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.resolver.element_resolver;
 
 import 'dart:collection';
 
 import 'ast.dart';
 import 'element.dart';
@@ skipping 573 matching lines @@
     }
     Element staticElement;
     Element propagatedElement;
     DartType staticType = null;
     DartType propagatedType = null;
     if (target == null) {
       staticElement = _resolveInvokedElement(methodName);
       propagatedElement = null;
     } else if (methodName.name == FunctionElement.LOAD_LIBRARY_NAME &&
         _isDeferredPrefix(target)) {
+      if (node.operator.type == sc.TokenType.QUESTION_PERIOD) {
+        _resolver.reportErrorForNode(
+            CompileTimeErrorCode.PREFIX_IDENTIFIER_NOT_FOLLOWED_BY_DOT, target,
+            [(target as SimpleIdentifier).name]);
+      }
       LibraryElement importedLibrary = _getImportedLibrary(target);
       methodName.staticElement = importedLibrary.loadLibraryFunction;
       return null;
     } else {
       staticType = _getStaticType(target);
       propagatedType = _getPropagatedType(target);
       //
       // If this method invocation is of the form 'C.m' where 'C' is a class,
       // then we don't call resolveInvokedElement(...) which walks up the class
       // hierarchy, instead we just look for the member in the type only.  This
@@ skipping 2202 matching lines @@
 
   @override
   Element get staticElement => null;
 
   @override
   accept(AstVisitor visitor) => null;
 
   @override
   void visitChildren(AstVisitor visitor) {}
 }