This slightly long statement works for me:
import static android.support.test.espresso.assertion.ViewAssertions.matches;
import static android.support.test.espresso.matcher.RootMatchers.withDecorView;
import static android.support.test.espresso.matcher.ViewMatchers.isDisplayed;
import static android.support.test.espresso.matcher.ViewMatchers.withText;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.not;
....
onView(withText(R.string.TOAST_STRING)).inRoot(withDecorView(not(is(getActivity().getWindow().getDecorView())))).check(matches(isDisplayed()));
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…