getEnclosingFinallyMap() {
    let enclosingMap = Object.create(null);
    addCatchOrFinallyStates(TryState.Kind.FINALLY, enclosingMap,
                            this.exceptionBlocks);
    return enclosingMap;
  }
Return a map from the states in the machine to their nearest enclosing
finally.
@return {Object} map of state IDs to FinallyState.