Support loading of HTML imports using @HtmlImport (#8301)
Note that not all browsers yet support HTML imports. If a polyfill
is needed to load HTML imports, it must be loaded before HTML Imports
can be loaded. There is no automatic loading of any polyfills.
Remove custom preloading support and load scripts using async=false (#8291)
When using async='false' for scripts created by scripts
the execution order is guaranteed to be the same as the order the
script tags are created
Fixes #5339, #3631