Currently dynamically added imports behaves as if it is @async applied.
This should change. It should block following imports, in terms of script execution.

For example:
----
<link rel=import href=a.html>
<script>
var currentDocument = document.currentScript.ownerDocument;
var b = currentDocument.createElement("link");
b.rel = "import";
b.href = "b.html";
currentDocument.appendChild(b);
var c = currentDocument.createElement("link");
c.rel = "import";
c.href = "c.html";
currentDocument.appendChild(c);
</script>
<link rel=import href=d.html>
----

In this case, scripts in imports should be executed in order of [a.html, b.html, c.html, d.html]
instead of current [a.html, (probably)d.html, (b or c).html, (b or c).html].

What's the motivation?

We need this because in some case we want to dynamically load a set of imports has dependency in it. For example we might want to lazily load plugin-base.html and plugin-hmm.html, where plugin-hmm.html depends on plugin-base.html.