Avoid Array.prototype.slice.{call,apply}(arguments, ...) in hot code.

34 consecutive chewbacca runs says this is a 3-16% improvement.
Median: 14.4%
Average: 11.2%

lib/Unexpected.js

@@ -487,7 +487,13 @@ Unexpected.prototype.fail = function (arg) {
             error[key] = additionalProperties[key];
         });
     } else {
-        var placeholderArgs = Array.prototype.slice.call(arguments, 1);
+        var placeholderArgs;
+        if (arguments.length > 0) {
+            placeholderArgs = new Array(arguments.length - 1);
+            for (var i = 1 ; i < arguments.length ; i += 1) {
+                placeholderArgs[i - 1] = arguments[i];
+            }
+        }
         error.errorMode = 'bubble';
         error.output = function (output) {
             var message = arg ? String(arg) : 'Explicit failure';
@@ -496,7 +502,7 @@ Unexpected.prototype.fail = function (arg) {
                 var match = placeholderRegexp.exec(token);
                 if (match) {
                     var index = match[1];
-                    if (index in placeholderArgs) {
+                    if (placeholderArgs && index in placeholderArgs) {
                         var placeholderArg = placeholderArgs[index];
                         if (placeholderArg && placeholderArg.isMagicPen) {
                             output.append(placeholderArg);
@@ -1020,7 +1026,10 @@ Unexpected.prototype.expect = function expect(subject, testDescriptionString) {
                 return Boolean(flags[flag]) !== Boolean(negate) ? flag + ' ' : '';
             }).trim();
 
-            var args = Array.prototype.slice.call(arguments, 2);
+            var args = new Array(arguments.length - 2);
+            for (var i = 2 ; i < arguments.length ; i += 1) {
+                args[i - 2] = arguments[i];
+            }
             return wrappedExpect.callInNestedContext(function () {
                 return executeExpect(subject, testDescriptionString, args);
             });
@@ -1061,7 +1070,10 @@ Unexpected.prototype.expect = function expect(subject, testDescriptionString) {
         return oathbreaker(assertionRule.handler.apply(wrappedExpect, [wrappedExpect, subject].concat(args)));
     }
 
-    var args = Array.prototype.slice.call(arguments, 2);
+    var args = new Array(arguments.length - 2);
+    for (var i = 2 ; i < arguments.length ; i += 1) {
+        args[i - 2] = arguments[i];
+    }
     try {
         var result = executeExpect(subject, testDescriptionString, args);
         if (isPendingPromise(result)) {