Change String.prototype.concat to be more like similar functions.

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 9719df4c288f2a53af851574fda85180fa0b27e8..0d5ed0fc4297c49b0f79917de5a9e6b6a2758c4d 100644
--- a/src/string.js
+++ b/src/string.js
@@ -61,13 +61,13 @@ function StringCharCodeAt(pos) {
 
 
 // ECMA-262, section 15.5.4.6
-function StringConcat() {
+function StringConcat(other /* and more */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.concat");
 
   var len = %_ArgumentsLength();
   var this_as_string = TO_STRING_INLINE(this);
   if (len === 1) {
-    return this_as_string + %_Arguments(0);
+    return this_as_string + other;
   }
   var parts = new InternalArray(len + 1);
   parts[0] = this_as_string;
@@ -78,9 +78,6 @@ function StringConcat() {
   return %StringBuilderConcat(parts, len + 1, "");
 }
 
-// Match ES3 and Safari
-%FunctionSetLength(StringConcat, 1);
-
 
 // ECMA-262 section 15.5.4.7
 function StringIndexOfJS(pattern /* position */) {  // length == 1