Add assertion to make sure analyzer sets NullSafetyUnderstandingFlag.

Change-Id: Ic0e3681117cb9783be6ee5b92ba8f79978d6c9bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/167361
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart

@@ -6,6 +6,7 @@ import 'package:analyzer/dart/analysis/declared_variables.dart';
 import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/null_safety_understanding_flag.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/src/dart/analysis/file_state.dart';
@@ -107,6 +108,10 @@ class LibraryAnalyzer {
 
   /// Compute analysis results for all units of the library.
   Map<FileState, UnitAnalysisResult> analyzeSync() {
+    // Analyzer understands null safety, so it should set
+    // NullSafetyUnderstandingFlag.
+    assert(NullSafetyUnderstandingFlag.isEnabled);
+
     timerLibraryAnalyzer.start();
     Map<FileState, CompilationUnit> units = {};