Properly skip Set tests in browsers that don't really support it

spec/helpers/BrowserFlags.js

@@ -20,4 +20,23 @@
     return /Firefox\/([0-9]{0,})/.exec(userAgent);
   });
 
+  function hasFunctioningSets() {
+    if (typeof Set !== 'undefined') { return false; }
+
+    try {
+      var s = new Set([4]);
+      if (s.size !== 1) { return false; }
+      if (s.values().next().value !== 4) { return false; }
+      return true;
+    } catch(e) {
+      return false;
+    }
+  }
+
+  env.requireFunctioningSets = function() {
+    if (!hasFunctioningSets()) {
+      pending("Browser has incomplete or missing support for Sets");
+    }
+  };
+
 })(jasmine.getEnv());