WASHINGTON — Two key undecided senators signaled Thursday that they are satisfied with the F.B.I.’s investigation into allegations of sexual assault against Judge Brett M. Kavanaugh, and Senate Republican leaders were increasingly confident that he would be confirmed to the Supreme Court. Senators Jeff Flake of Arizona and Susan Collins of Maine did not say that they will vote for Judge Kavanaugh, President Trump’s second Supreme Court nominee. But after a closed-door briefing in which Republicans were told that no witnesses corroborated the accounts of Judge Kavanaugh’s main accusers, both made positive remarks. A yes vote from both would secure...