Fix operations with fragments in the Webpack loader for IE 11 (#190)

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/label bug

I was using version 2.9.2 with the Webpack loader. On IE 11, I saw the following error:

    Fragment was XXX used, but not defined

It turns out that on line 133 of loader.js, we have `var newRefs = new Set(opRefs);` and [IE 11 doesn't support "new Set(iterable)"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set), so I rewrite it to add the members of opRefs to newRefs one by one using `forEach`, which resovles the issue.

loader.js

@@ -130,7 +130,13 @@ module.exports = function(source) {
       // it or the fragments it references
       var opRefs = definitionRefs[operationName] || new Set();
       var allRefs = new Set();
-      var newRefs = new Set(opRefs);
+      var newRefs = new Set();
+
+      // IE 11 doesn't support "new Set(iterable)", so we add the members of opRefs to newRefs one by one
+      opRefs.forEach(function(refName) {
+        newRefs.add(refName);
+      });
+
       while (newRefs.size > 0) {
         var prevRefs = newRefs;
         newRefs = new Set();