Restore weak mode errors on empty returns in Null functions

pkg/analyzer/test/generated/strong_mode_test.dart

Index: pkg/analyzer/test/generated/strong_mode_test.dart
diff --git a/pkg/analyzer/test/generated/strong_mode_test.dart b/pkg/analyzer/test/generated/strong_mode_test.dart
index ac834e6460a3b44a0ae208e41725c85702cc73cb..b647305528fb4dbecb13ef1263aa02586e9527dd 100644
--- a/pkg/analyzer/test/generated/strong_mode_test.dart
+++ b/pkg/analyzer/test/generated/strong_mode_test.dart
@@ -1143,7 +1143,7 @@ class StrongModeLocalInferenceTest extends ResolverTestCase {
     MethodInvocation invoke = await _testFutureOr(r'''
     FutureOr<T> mk<T>(Future<T> x) => x;
     Future<int> f;
-    test() => f.then((int x) {});
+    test() => f.then((int x) {return null;});
     ''');
     _isFunction2Of(_isInt, _isNull)(
         invoke.argumentList.arguments[0].staticType);
@@ -1198,7 +1198,7 @@ class StrongModeLocalInferenceTest extends ResolverTestCase {
     MethodInvocation invoke = await _testFutureOr(r'''
     FutureOr<T> mk<T>(Future<T> x) => x;
     Future<int> f;
-    test() => f.then<Null>((int x) {});
+    test() => f.then<Null>((int x) { return null;});
     ''');
     _isFunction2Of(_isInt, _isNull)(
         invoke.argumentList.arguments[0].staticType);