Do not switch to two-byte string in String.fromCharCode if avoidable.

src/js/string.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
@@ skipping 755 matching lines @@
 function StringTrimRight() {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.trimRight");
 
   return %StringTrim(TO_STRING(this), false, true);
 }
 
 
 // ECMA-262, section 15.5.3.2
 function StringFromCharCode(code) {
   var n = %_ArgumentsLength();
-  if (n == 1) {
-    if (!%_IsSmi(code)) code = TO_NUMBER(code);
-    return %_StringCharFromCode(code & 0xffff);
-  }
+  if (n == 1) return %_StringCharFromCode(code & 0xffff);
 
   var one_byte = %NewString(n, NEW_ONE_BYTE_STRING);
   var i;
   for (i = 0; i < n; i++) {
-    var code = %_Arguments(i);
-    if (!%_IsSmi(code)) code = TO_NUMBER(code) & 0xffff;
-    if (code < 0) code = code & 0xffff;
+    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);
-  for (var j = 0; i < n; i++, j++) {
-    var code = %_Arguments(i);
-    if (!%_IsSmi(code)) code = TO_NUMBER(code) & 0xffff;
+  %_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;
 }
 
 
 // ES6 draft, revision 26 (2014-07-18), section B.2.3.2.1
 function HtmlEscape(str) {
   return %_Call(StringReplace, TO_STRING(str), /"/g, "&quot;");
 }
@@ skipping 362 matching lines @@
   to.StringLastIndexOf = StringLastIndexOfJS;
   to.StringMatch = StringMatchJS;
   to.StringReplace = StringReplace;
   to.StringSlice = StringSlice;
   to.StringSplit = StringSplitJS;
   to.StringSubstr = StringSubstr;
   to.StringSubstring = StringSubstring;
 });
 
 })