pushForkContext(forkLeavingPath) {
    this.forkContext = ForkContext.newEmpty(this.forkContext, forkLeavingPath);

    return this.forkContext;
  }
Creates and stacks new forking context.
@param {boolean} forkLeavingPath A flag which shows being in a
  "finally" block.
@returns {ForkContext} The created context.