make a local variable that points to Array.prototype since the former is not min'd down.

Won't effect gzip size, but reduces min size.  Saves about:

15 bytes per instance * 13 instances ≅ 200 bytes

underscore.js

@@ -30,28 +30,29 @@
   // Export the Underscore object for CommonJS.
   if (typeof exports !== 'undefined') exports._ = _;
 
+  // Save bytes in the minified (but not gzipped) version:
+  var ArrayPrototype = Array.prototype;
+  
   // Create quick reference variables for speed access to core prototypes.
-  var slice                 = Array.prototype.slice,
-      unshift               = Array.prototype.unshift,
+  var slice                 = ArrayPrototype.slice,
+      unshift               = ArrayPrototype.unshift,
       toString              = Object.prototype.toString,
       hasOwnProperty        = Object.prototype.hasOwnProperty,
       propertyIsEnumerable  = Object.prototype.propertyIsEnumerable;
 
-  // All native implementations we hope to use are declared here, 
-  // mostly Array.prototype methods.
-  //
+  // All native implementations we hope to use are declared here.
   // 'native' is on the long list of reserved words.  
   // Ok to use as a property name, though jsLint doesn't agree.
   var Native = _['native'] = {
-    forEach:        Array.prototype.forEach,
-    map:            Array.prototype.map,
-    reduce:         Array.prototype.reduce,
-    reduceRight:    Array.prototype.reduceRight,
-    filter:         Array.prototype.filter,
-    every:          Array.prototype.every,
-    some:           Array.prototype.some,
-    indexOf:        Array.prototype.indexOf,
-    lastIndexOf:    Array.prototype.lastIndexOf,
+    forEach:        ArrayPrototype.forEach,
+    map:            ArrayPrototype.map,
+    reduce:         ArrayPrototype.reduce,
+    reduceRight:    ArrayPrototype.reduceRight,
+    filter:         ArrayPrototype.filter,
+    every:          ArrayPrototype.every,
+    some:           ArrayPrototype.some,
+    indexOf:        ArrayPrototype.indexOf,
+    lastIndexOf:    ArrayPrototype.lastIndexOf,
     isArray:        Array.isArray,
     keys:           Object.keys
   };
@@ -688,7 +689,7 @@
 
   // Add all mutator Array functions to the wrapper.
   each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
-    var method = Array.prototype[name];
+    var method = ArrayPrototype[name];
     wrapper.prototype[name] = function() {
       method.apply(this._wrapped, arguments);
       return result(this._wrapped, this._chain);
@@ -697,7 +698,7 @@
 
   // Add all accessor Array functions to the wrapper.
   each(['concat', 'join', 'slice'], function(name) {
-    var method = Array.prototype[name];
+    var method = ArrayPrototype[name];
     wrapper.prototype[name] = function() {
       return result(method.apply(this._wrapped, arguments), this._chain);
     };