Error mangling identifier in try-catch when there's same-named label

[kangax]

This:

function f(a) {
  try {
    a: {
      console.log(a);
    }
  } catch ($a) { }
}

becomes:

"use strict";function f(b){try{a:console.log(a)}catch(c){}}

Notice console.log(a) instead of console.log(b).