Disallow assigning null to non-nullable variables

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index ca3eebc4ec9760aaf52747cb47c56815ba964f16..55f2b9c5a94e4f57b8e950e536047601a5c4eda0 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -292,6 +292,9 @@ class AnalysisContextImpl implements InternalAnalysisContext {
             ? this._options.implicitCasts != options.implicitCasts
             : false) ||
         ((options is AnalysisOptionsImpl)
+            ? this._options.nonnullableTypes != options.nonnullableTypes
+            : false) ||
+        ((options is AnalysisOptionsImpl)
             ? this._options.implicitDynamic != options.implicitDynamic
             : false) ||
         this._options.enableStrictCallChecks !=
@@ -326,6 +329,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     if (options is AnalysisOptionsImpl) {
       this._options.strongModeHints = options.strongModeHints;
       this._options.implicitCasts = options.implicitCasts;
+      this._options.nonnullableTypes = options.nonnullableTypes;
       this._options.implicitDynamic = options.implicitDynamic;
     }
     if (needsRecompute) {