The Netherlands on Wednesday took a startling turn in national elections with the potential to ripple through Europe, as Dutch voters threw most support behind the party of a far-right icon with an incendiary reputation who had campaigned on an anti-immigrant platform. Geert Wilders, a political provocateur long known for his anti-Islam and anti-Europe stances, appeared poised to come out significantly ahead with the most parliamentary seats, according to some early results and exit polls, which were expected to be dependable, especially given the margin of victory they indicated. “The Dutch voter has spoken,” Mr. Wilders said in a speech...