Adding a test for importer callback order

[beatak]

This pull request is to add a test case for sass/libsass#1262 and this test is currently failing. I have chatted with @xzyfer and he thinks it's a bug.

Besides the main point (the callback order is not DFS), I also found that the importer won't callback for the nested nodes.

This test add index.scss and tree is as follows:

                   +-----+            
                   |index|            
                   +--+--+            
                      |               
         +-------------------------+  
        +-+        +------+       +-+ 
        |a|        |common|       |b| 
        +++        +--+---+       +++ 
         |            |            |  
   +-----+--+     +---+----+       |  
+------+  +--+  +----+ +------+   ++-+
|common|  |a1|  |vars| |struct|   |b1|
+--+---+  +--+  +----+ +------+   +--+
   |                                  
  +--------+                          
+----+ +------+                       
|vars| |struct|                       
+----+ +------+                       

Actually, node-sass calls nodes in the order of [a, common, b, common, a1, vars, struct, b1] instead of [a, common, vars, struct, a1, common, vars, struct, b, b1] Not only the order is wrong, but also vars and struct didn't get called twice.

[xzyfer]

Great, I'll dig into this with you tomorrow. I looked into it briefly for sass/libsass#1262 but was not able to reproduce as per sass/libsass#1262 (comment).

[xzyfer]

I wonder if you're running into #894

[xzyfer]

I've been able to confirm this is an issue with custom importers only. Libsass is resolving @imports depth-first, but custom importers are being call breadth-first.

[xzyfer]

Superseded by #1420