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

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

Index: pkg/analyzer/lib/src/generated/element_resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/element_resolver.dart b/pkg/analyzer/lib/src/generated/element_resolver.dart
index dcf6c51194f0f90b320188de3c9f826b0caa2826..98635d306cb1ba19fcdf896d3b39215e62e79312 100644
--- a/pkg/analyzer/lib/src/generated/element_resolver.dart
+++ b/pkg/analyzer/lib/src/generated/element_resolver.dart
@@ -591,6 +591,11 @@ class ElementResolver extends SimpleAstVisitor<Object> {
       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;