Optimize concat for one argument (two if you count 'this').

Review URL: http://codereview.chromium.org/584006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3815 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/string.js

@@ -87,14 +87,18 @@ function StringCharCodeAt(pos) {
 
 // ECMA-262, section 15.5.4.6
 function StringConcat() {
-  var len = %_ArgumentsLength() + 1;
-  var parts = new $Array(len);
-  parts[0] = IS_STRING(this) ? this : ToString(this);
-  for (var i = 1; i < len; i++) {
-    var part = %_Arguments(i - 1);
-    parts[i] = IS_STRING(part) ? part : ToString(part);
+  var len = %_ArgumentsLength();
+  var this_as_string = IS_STRING(this) ? this : ToString(this);
+  if (len === 1) {
+    return this_as_string + %_Arguments(0);
   }
-  return %StringBuilderConcat(parts, len, "");
+  var parts = new $Array(len + 1);
+  parts[0] = this_as_string;
+  for (var i = 0; i < len; i++) {
+    var part = %_Arguments(i);
+    parts[i + 1] = IS_STRING(part) ? part : ToString(part);
+  }
+  return %StringBuilderConcat(parts, len + 1, "");
 }
 
 // Match ES3 and Safari