[builtins] Rewrite String.fromCharCode w/o %_Arguments.

This is the very last use of %_Arguments and %_ArgumentsLength,
and also the main blocker for not inlining builtins.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1681403006

Cr-Commit-Position: refs/heads/master@{#33939}

src/js/string.js

@@ -559,28 +559,14 @@ function StringTrimRight() {
 
 
 // ECMA-262, section 15.5.3.2
-function StringFromCharCode(code) {
-  var n = %_ArgumentsLength();
-  if (n === 1) return %_StringCharFromCode(code & 0xffff);
-
-  var one_byte = %NewString(n, NEW_ONE_BYTE_STRING);
-  var i;
-  for (i = 0; i < n; i++) {
-    code = %_Arguments(i) & 0xffff;
-    if (code > 0xff) break;
-    %_OneByteSeqStringSetChar(i, code, one_byte);
-  }
-  if (i == n) return one_byte;
-  one_byte = %TruncateString(one_byte, i);
-
-  var two_byte = %NewString(n - i, NEW_TWO_BYTE_STRING);
-  %_TwoByteSeqStringSetChar(0, code, two_byte);
-  i++;
-  for (var j = 1; i < n; i++, j++) {
-    code = %_Arguments(i) & 0xffff;
-    %_TwoByteSeqStringSetChar(j, code, two_byte);
-  }
-  return one_byte + two_byte;
+function StringFromCharCode(_) {  // length == 1
+  "use strict";
+  var s = "";
+  var n = arguments.length;
+  for (var i = 0; i < n; ++i) {
+    s += %_StringCharFromCode(arguments[i] & 0xffff);
+  }
+  return s;
 }