src/codegeneration/ClassTransformer.js
function classCall(func, object, staticObject, superClass) {
  if (superClass) {
    return parseExpression
        `($traceurRuntime.createClass)(${func}, ${object}, ${staticObject},
                                       ${superClass})`;
  }
  return parseExpression
      `($traceurRuntime.createClass)(${func}, ${object}, ${staticObject})`;
}
function classCall(func, object, staticObject, superClass) {
  if (superClass) {
    return parseExpression
        `($traceurRuntime.createClass)(${func}, ${object}, ${staticObject},
                                       ${superClass})`;
  }
  return parseExpression
      `($traceurRuntime.createClass)(${func}, ${object}, ${staticObject})`;
}
Maximally minimal classes  http://wiki.ecmascript.org/doku.php?id=strawman:maximally_minimal_classes  This transforms class declarations and class expressions.  class C extends B { constructor(x) { super(); } method() { super.m(); } }  =>  var C = function(x) { $__superCall(this, $C.prototype, 'constructor', []); }; var $C = $traceurRuntime.createClass(C, { method: function() { $traceurRuntime.superCall(this, $C.prototype, 'm', []); } }, {}, B);