Keep fragment in the template's document during clone

The template's document doesn't have a browsing context, so
custom elements aren't upgraded. This change defers upgrading
any custom elements inside the template until after the
treewalker that links the Parts to the DOM.
This is important because custom elements can modify their DOM
at upgrade time (e.g. when using ShadyDOM).

Fixes #231

src/lit-html.ts

@@ -729,7 +729,10 @@ export class TemplateInstance {
   }
 
   _clone(): DocumentFragment {
-    const fragment = document.importNode(this.template.element.content, true);
+    // Clone the node, rather than importing it, to keep the fragment in the
+    // template's document. This leaves the fragment inert so custom elements
+    // won't upgrade until after the main document adopts the node.
+    const fragment = this.template.element.content.cloneNode(true);
     const parts = this.template.parts;
 
     if (parts.length > 0) {