Avoid accessing arguments array without length check.

Accessing elements after the end of the arguments array triggers
deoptimization of the entire function on v8, so avoid doing so.  Also
tweak all the checks into the same standard form for consistency.

es6-shim.js

@@ -1096,7 +1096,9 @@
 
   var ArrayPrototypeShims = {
     copyWithin: function copyWithin(target, start) {
-      var end = arguments[2]; // copyWithin.length must be 2
+      // copyWithin.length must be 2, so we can't add `end` to the arguments
+      // directly.
+      var end = arguments.length > 2 ? arguments[2] : void 0;
       var o = ES.ToObject(this);
       var len = ES.ToLength(o.length);
       var relativeTarget = ES.ToInteger(target);
@@ -1243,7 +1245,7 @@
   if (!arrayFromHandlesUndefinedMapFunction) {
     var origArrayFrom = Array.from;
     overrideNative(Array, 'from', function from(items) {
-      if (arguments.length > 0 && typeof arguments[1] !== 'undefined') {
+      if (arguments.length > 1 && typeof arguments[1] !== 'undefined') {
         return ES.Call(origArrayFrom, this, arguments);
       } else {
         return _call(origArrayFrom, this, items);
@@ -2442,7 +2444,7 @@
         if (!ES.IsPromise(promise)) { throw new TypeError('not a promise'); }
         var C = ES.SpeciesConstructor(promise, Promise);
         var resultCapability;
-        var returnValueIsIgnored = (arguments[2] === PROMISE_FAKE_CAPABILITY);
+        var returnValueIsIgnored = (arguments.length > 2 && arguments[2] === PROMISE_FAKE_CAPABILITY);
         if (returnValueIsIgnored && C === Promise) {
           resultCapability = PROMISE_FAKE_CAPABILITY;
         } else {
@@ -3302,7 +3304,7 @@
       if (!ES.IsConstructor(constructor)) {
         throw new TypeError('First argument must be a constructor.');
       }
-      var newTarget = arguments.length < 3 ? constructor : arguments[2];
+      var newTarget = arguments.length > 2 ? arguments[2] : constructor;
       if (!ES.IsConstructor(newTarget)) {
         throw new TypeError('new.target must be a constructor.');
       }