Feral fairies. Anyone afraid of a sugar overdose had nothing to worry about at the English National Opera’s fiercely odd production of Britten’s A Midsummer Night’s Dream. This is a company that often traffics between radical and gimmicky, every once in a while being capable of acts of metamorphosis. In this case the director, Christopher Alden, seems to have taken his cue from the first sound we hear: slithery glissandos in the lower strings that introduce the fairy world not with whimsy and a twinkle but a wave of sea-sick nausea. In a daring move, the entire production metastasizes from the queasiness of that sound. The initial effect was to travesty Shakespeare’s magical comedy — audience members who stalked up the aisle before the first act ended clearly didn’t appreciate such high-handedness — but the music has never sounded so disturbing, or so convincing.