Add Firefox support to labs

Add support for Firefox.

The code was using the Set class's "difference" method.
This is in Firefox nightly, but is not yet released.
So we add a few lines to compute it ourselves. Now it works on Firefox.

Signed-off-by: David A. Wheeler <[email protected]>

Author: david-a-wheeler

docs/labs/checker.js

@@ -66,6 +66,18 @@ function escapeHTML(unsafe) {
             .replace(/\'/g, "'"));
 }
 
+/* Compute Set difference lhs \ rhs.
+ * @lhs - Set to start with
+ * @rhs - Set to remove from the lhs
+ * Set difference is in Firefox nightly, but is not yet released.
+ * So we compute it ourselves. This is equivalent to lhs.difference(rhs)
+ */
+function setDifference(lhs, rhs) {
+    let lhsArray = Array.from(lhs);
+    let result = lhsArray.filter((x) => {!rhs.has(x)});
+    return new Set(result);
+}
+
 /*
  * Show debug output in debug region and maybe via alert box
  * @debugOutput - the debug information to show
@@ -253,7 +265,7 @@ function processHints(requestedHints) {
 
         // Complain about unknown fields
         let usedFields = new Set(Object.keys(hint));
-        let forbiddenFields = usedFields.difference(allowedHintFields);
+        let forbiddenFields = setDifference(usedFields, allowedHintFields);
         if (forbiddenFields.size != 0) {
             showDebugOutput(
                 `Unknown field(s) in hint[${i}]: ` +
@@ -306,7 +318,7 @@ function processInfo(configurationInfo) {
         'hints', 'successes', 'failures', 'correct', 'expected',
         'debug']);
     let usedFields = new Set(Object.keys(info));
-    let forbiddenFields = usedFields.difference(allowedInfoFields);
+    let forbiddenFields = setDifference(usedFields, allowedInfoFields);
     if (forbiddenFields.size != 0) {
         showDebugOutput(
             `Unknown field(s) in info: ` +