Fix compatibility bugs with Firefox 25, 30, 31.

Shim `Array#find` and `Array#findIndex` if they don't match ES6 spec's
treatment of holes.  (Be sure to do this test after `Number` has been
shimmed, and we've already eliminated a premature use of `Math.sign`.)

Broaden the permissible values of `Math.cosh` (Firefox 31 on Linux/x86
shows a difference here.)

Fix an overly-specific test for `Symbol.iterator` (for compatibility,
we use `'@@iterator'` as the name of our iterator on Firefox, in the
absence of `Symbol.iterator`).

The `defineProperties` method doesn't work if we are using
`Symbol.iterator` as the name for the iterator method.  Only newer
browsers define enough of `Symbol` to encounter this problem.  Be
careful when shimming iterators.

es6-shim.js

@@ -116,6 +116,11 @@
       var o = {};
       o[$iterator$] = impl;
       defineProperties(prototype, o);
+      /* jshint notypeof: true */
+      if (!prototype[$iterator$] && typeof($iterator$)==='symbol') {
+        // implementations are buggy when $iterator$ is a Symbol
+        prototype[$iterator$] = impl;
+      }
     };
 
     // taken directly from https://github.com/ljharb/is-arguments/blob/master/index.js
@@ -667,7 +672,7 @@
     });
     addIterator(ArrayIterator.prototype);
 
-    var arrayPrototypeShims = {
+    var ArrayPrototypeShims = {
       copyWithin: function(target, start) {
         var end = arguments[2]; // copyWithin.length must be 2
         var o = ES.ToObject(this);
@@ -754,23 +759,7 @@
         return new ArrayIterator(this, "entry");
       }
     };
-    defineProperties(Array.prototype, arrayPrototypeShims);
-
-    var handlesSparseArrays = function (method) {
-      var count = 0;
-      method.call([1, ,], function (item) { count += 1; });
-      return count === 2;
-    };
-
-    // Firefox >= 25 (up to 31 at least) does not handle sparse arrays properly
-    // for Array#find and Array#findIndex
-    if (!handlesSparseArrays(Array.prototype.find)) {
-      defineProperty(Array.prototype, 'find', arrayPrototypeShims.find, true);
-    }
-    if (!handlesSparseArrays(Array.prototype.findIndex)) {
-      defineProperty(Array.prototype, 'findIndex', arrayPrototypeShims.findIndex, true);
-    }
-
+    defineProperties(Array.prototype, ArrayPrototypeShims);
     addIterator(Array.prototype, function() { return this.values(); });
     // Chrome defines keys/values/entries on Array, but doesn't give us
     // any way to identify its iterator.  So add our own shimmed field.
@@ -811,6 +800,17 @@
 
     });
 
+    // Work around bugs in Array#find and Array#findIndex -- early
+    // implementations skipped holes in sparse arrays. (Note that the
+    // implementations of find/findIndex indirectly use shimmed
+    // methods of Number, so this test has to happen down here.)
+    if (![,1].find(function(item,idx) { return idx===0; })) {
+      defineProperty(Array.prototype, 'find', ArrayPrototypeShims.find, true);
+    }
+    if ([,1].findIndex(function(item,idx) { return idx===0; }) !== 0) {
+      defineProperty(Array.prototype, 'findIndex', ArrayPrototypeShims.findIndex, true);
+    }
+
     if (supportsDescriptors) {
       defineProperties(Object, {
         getPropertyDescriptor: function(subject, name) {

test/array.js

@@ -1,27 +1,16 @@
 var runArrayTests = function() {
-  var getIteratorPropertyName = function() {
-    var iterator;
-    if (typeof Symbol === 'function' && Symbol.iterator) {
-      iterator = Symbol.iterator;
-    } else {
-      iterator = '_es6shim_iterator_';
-    }
-    // Firefox ships a partial implementation using the name @@iterator.
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=907077#c14
-    // So use that name if we detect it.
-    if (Set && typeof new Set()['@@iterator'] === 'function') {
-      iterator = '@@iterator';
-    }
-    return iterator;
-  };
-
   describe('Array', function() {
     var list = [5, 10, 15, 20];
 
     describe('@@iterator', function() {
       it('uses Symbol.iterator if available', function() {
         var a = [];
-        var iterator = getIteratorPropertyName();
+        var iterator;
+        if (typeof Symbol === 'function' && Symbol.iterator) {
+          iterator = Symbol.iterator;
+        } else {
+          return;
+        }
         expect(a[iterator]()).to.eql(a.values());
       });
     });

test/math.js

@@ -149,7 +149,8 @@ describe('Math', function() {
 
     it('should be correct', function() {
       // Overridden precision values here are for Chrome, as of v25.0.1364.172
-      expect(Math.cosh(12)).to.almostEqual(81377.39571257407, 3e-11);
+      // Broadened slightly for Firefox 31
+      expect(Math.cosh(12)).to.almostEqual(81377.39571257407, 9e-11);
       expect(Math.cosh(22)).to.almostEqual(1792456423.065795780980053377, 1e-5);
       expect(Math.cosh(-10)).to.almostEqual(11013.23292010332313972137);
       expect(Math.cosh(-23)).to.almostEqual(4872401723.1244513000, 1e-5);