Fix last failing _tests.

Author: ykmnkmi

_tests/test/core/linker/integration/on_push_test.dart

@@ -72,7 +72,7 @@ void main() {
       cmpElement.children
           .item(2)!
           .children
-          .item(1)!
+          .item(0)!
           .dispatchEvent(MouseEvent('click'));
     });
     expect(cmp.numberOfChecks, 4);

_tests/test/core/linker/integration/outputs_test.dart

@@ -61,8 +61,9 @@ void main() {
     final inputPrevent =
         testFixture.rootElement.children.item(0) as HTMLInputElement;
     final inputNoPrevent =
-        testFixture.rootElement.children.item(0) as HTMLInputElement;
-    final clickPrevent = MouseEvent('click');
+        testFixture.rootElement.children.item(1) as HTMLInputElement;
+    // `true` by default in the `dart:html` Event contructor
+    final clickPrevent = MouseEvent('click', MouseEventInit(cancelable: true));
     final clickNoPrevent = MouseEvent('click');
     inputPrevent.dispatchEvent(clickPrevent);
     inputNoPrevent.dispatchEvent(clickNoPrevent);