[tests] Fix type of stream getter in test

The focus event stream is typed to be non-nullable. Updated the
return of the function passed to reduce in the "_2" version as
well to keep them consistent.

Change-Id: Ic1ae2e027c184d751549c989d60924360139fd5c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154140
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

tests/lib/html/streams_test.dart

@@ -15,7 +15,7 @@ class StreamHelper {
   }
 
   Element get element => _a;
-  Stream<Event?> get stream => _a.onFocus;
+  Stream<Event> get stream => _a.onFocus;
 
   // Causes an event on a to be fired.
   void pulse() {
@@ -193,7 +193,7 @@ main() {
   });
 
   test('reduce', () {
-    stream.reduce((a, b) => null).then((_) {});
+    stream.reduce((a, b) => a).then((_) {});
   });
 
   test('fold', () {

tests/lib_2/html/streams_test.dart

@@ -189,7 +189,7 @@ main() {
   });
 
   test('reduce', () {
-    stream.reduce((a, b) => null).then((_) {});
+    stream.reduce((a, b) => a).then((_) {});
   });
 
   test('fold', () {