function bindBlock(env, scope, block, name='default') {
  scope.blocks[name] = block;
}
Host Hook: bindBlock

  @param {Environment} env
  @param {Scope} scope
  @param {Function} block

  Corresponds to entering a shadow template that was invoked by a block helper with
  `yieldIn`.

  This hook is invoked with an opaque block that will be passed along
  to the shadow template, and inserted into the shadow template when
  `{{yield}}` is used. Optionally provide a non-default block name
  that can be targeted by `{{yield to=blockName}}`.