v3.1.6

• UIRoot: new registerComponents parameter, the components that markup can create by tag (<ui-button>,
  <ui-document>, <ui-dialog>...) to register when the first root is constructed. Without it all of them are
  registered, as before (the new static UIRoot.registerAllComponents). Whatever is registered is compiled in, so
  an app that writes none of those tags can pass () {}, or register only those it uses (UIDialog.register):
  <ui-document> alone brings in the markdown package. Every UIRoot of the app must pass it. In a real app
  (a game whose only component, a dialog, is created in code) the dart2js output went from 474 KiB gzipped to
  389 KiB, 18% smaller.
• Tests:
  • bones_ui_root_register_components_test.dart: a root registering only UIDialog has <ui-dialog> and none of
    the others; the registration runs once, before the first render; a registered tag becomes its component, an
    unregistered one (<ui-document>) stays a plain element, and a component created in code needs none.
  • bones_ui_root_register_all_components_test.dart: without registerComponents every tag component is
    registered, and <ui-document> renders its markdown.
  • register_components_tree_shaking_test.dart (VM): compiles minimal apps (test/register_components/) with
    dart compile js -O2. Registering none leaves every component and the markdown package out (about 30%
    smaller), registering UIDialog alone keeps only it, and one root without registerComponents keeps them all.