lookup(name) {
    var block = this.currentFrame.block
      , val
      , ret;

    do {
      var frame = this.getBlockFrame(block);
      if (frame && (val = frame.lookup(name))) {
        return val;
      }
    } while (block = block.parent);
  }
Lookup the given local variable `name`, relative
to the lexical scope of the current frame's `Block`.

When the result of a lookup is an identifier
a recursive lookup is performed, defaulting to
returning the identifier itself.

@param {String} name
@return {Node}
@api private