fix($compile): work around Firefox DocumentFragment bug

DOM nodes passed to `compilationGenerator()` will eventually be wrapped
in `jqLite`, when the compilation actually happens. In Firefox 60+,
there seems to be a `DocumentFragment`-related bug that sometimes causes
the `childNodes` to be empty at the time the compilation happens.

This commit works around this bug by eagerly wrapping `childNodes` in
`jqLite`.

NOTE:
The wrapped nodes have references to their `DocumentFragment` container.
This is "by design", since we want to be able to traverse the nodes via
`nextSibling` (in order to correctly handle multi-element directives).

Once the nodes are compiled, they will be either moved to a new
container element or the `jqLite` wrapper is release making them
eligible for garbage collection. In both cases, the original
`DocumentFragment` container should be eligible for garbage collection
too.

Fixes angular#16607

Closes angular#16615

src/ng/compile.js

@@ -2619,11 +2619,12 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
               for (var slotName in slots) {
                 if (slots[slotName]) {
                   // Only define a transclusion function if the slot was filled
-                  slots[slotName] = compilationGenerator(mightHaveMultipleTransclusionError, slots[slotName].childNodes, transcludeFn);
+                  var slotCompileNodes = jqLite(slots[slotName].childNodes);
+                  slots[slotName] = compilationGenerator(mightHaveMultipleTransclusionError, slotCompileNodes, transcludeFn);
                 }
               }
 
-              $template = $template.childNodes;
+              $template = jqLite($template.childNodes);
             }
 
             $compileNode.empty(); // clear contents