Revert "Remove crude shim for ES6 Set. #904"

This reverts commit f2f6f1be6cbd5c1b21453832c31bea89cf319477.

src/classes/ShimSet.coffee

@@ -0,0 +1,18 @@
+class ShimSet
+  constructor: ->
+    @elements = {}
+    @size = 0
+  has: (value) ->
+    value of @elements
+  add: (value) ->
+    return if @elements[value]
+    @elements[value] = true
+    @size++
+  delete: (value) ->
+    return unless @elements[value]
+    delete @elements[value]
+    @size--
+
+window.Set = ShimSet unless 'Set' of window
+
+return ShimSet