Use numeric sort when removing dom-repeat instances

Fixes #2297

@@ -402,6 +402,10 @@
       return this.collection.getKey(a) - this.collection.getKey(b);
     },
 
+    _numericSort: function(a, b) {
+      return a - b;
+    },
+
     // Render method 2: incremental update using splices with user sort applied
     // ----
     // Removed/added keys are deduped, all removed rows are detached and pooled
@@ -438,7 +442,8 @@
       if (removedIdxs.length) {
         // Sort removed instances idx's then remove backwards,
         // so we don't invalidate instance index
-        removedIdxs.sort();
+        // use numeric sort, default .sort is alphabetic
+        removedIdxs.sort(this._numericSort);
         for (var i=removedIdxs.length-1; i>=0 ; i--) {
           var idx = removedIdxs[i];
           // Removed idx may be undefined if item was previously filtered out