Revert "First draft of removing native fallbacks for array extras."

This reverts commit 69e1677.

underscore.js

@@ -30,9 +30,15 @@
     hasOwnProperty   = ObjProto.hasOwnProperty;
 
   // All **ECMAScript 5** native function implementations that we hope to use
-  // are declared here. We don't use native implementations of the array extras
-  // methods introduced in ES5 -- at least until browsers optimize them.
+  // are declared here.
   var
+    nativeForEach      = ArrayProto.forEach,
+    nativeMap          = ArrayProto.map,
+    nativeReduce       = ArrayProto.reduce,
+    nativeReduceRight  = ArrayProto.reduceRight,
+    nativeFilter       = ArrayProto.filter,
+    nativeEvery        = ArrayProto.every,
+    nativeSome         = ArrayProto.some,
     nativeIndexOf      = ArrayProto.indexOf,
     nativeLastIndexOf  = ArrayProto.lastIndexOf,
     nativeIsArray      = Array.isArray,
@@ -70,7 +76,9 @@
   // Delegates to **ECMAScript 5**'s native `forEach` if available.
   var each = _.each = _.forEach = function(obj, iterator, context) {
     if (obj == null) return;
-    if (obj.length === +obj.length) {
+    if (nativeForEach && obj.forEach === nativeForEach) {
+      obj.forEach(iterator, context);
+    } else if (obj.length === +obj.length) {
       for (var i = 0, l = obj.length; i < l; i++) {
         if (iterator.call(context, obj[i], i, obj) === breaker) return;
       }
@@ -88,6 +96,7 @@
   _.map = _.collect = function(obj, iterator, context) {
     var results = [];
     if (obj == null) return results;
+    if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context);
     each(obj, function(value, index, list) {
       results.push(iterator.call(context, value, index, list));
     });
@@ -101,6 +110,10 @@
   _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
     var initial = arguments.length > 2;
     if (obj == null) obj = [];
+    if (nativeReduce && obj.reduce === nativeReduce) {
+      if (context) iterator = _.bind(iterator, context);
+      return initial ? obj.reduce(iterator, memo) : obj.reduce(iterator);
+    }
     each(obj, function(value, index, list) {
       if (!initial) {
         memo = value;
@@ -118,6 +131,10 @@
   _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
     var initial = arguments.length > 2;
     if (obj == null) obj = [];
+    if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
+      if (context) iterator = _.bind(iterator, context);
+      return initial ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
+    }
     var length = obj.length;
     if (length !== +length) {
       var keys = _.keys(obj);
@@ -154,6 +171,7 @@
   _.filter = _.select = function(obj, iterator, context) {
     var results = [];
     if (obj == null) return results;
+    if (nativeFilter && obj.filter === nativeFilter) return obj.filter(iterator, context);
     each(obj, function(value, index, list) {
       if (iterator.call(context, value, index, list)) results.push(value);
     });
@@ -174,6 +192,7 @@
     iterator || (iterator = _.identity);
     var result = true;
     if (obj == null) return result;
+    if (nativeEvery && obj.every === nativeEvery) return obj.every(iterator, context);
     each(obj, function(value, index, list) {
       if (!(result = result && iterator.call(context, value, index, list))) return breaker;
     });
@@ -187,6 +206,7 @@
     iterator || (iterator = _.identity);
     var result = false;
     if (obj == null) return result;
+    if (nativeSome && obj.some === nativeSome) return obj.some(iterator, context);
     each(obj, function(value, index, list) {
       if (result || (result = iterator.call(context, value, index, list))) return breaker;
     });