Implemented one-char cache lookup in generated code.

test/mjsunit/string-charat.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 33 matching lines @@
 assertEquals(116, s.charCodeAt(null));
 assertEquals(116, s.charCodeAt(void 0));
 assertEquals(116, s.charCodeAt(false));
 assertEquals(101, s.charCodeAt(true));
 assertEquals(116, s.charCodeAt(0));
 assertEquals(116, s.charCodeAt(3));
 assertEquals(116, s.charCodeAt(NaN));
 assertTrue(isNaN(s.charCodeAt(-1)));
 assertTrue(isNaN(s.charCodeAt(4)));
 
+// Make sure enough of the one-char string cache is filled.
+var alpha = ['@'];
+for (var i = 1; i < 128; i++) {
+  var c = String.fromCharCode(i);
+  alpha[i] = c.charAt(0);
+}
+var alphaStr = alpha.join("");
+
+// Now test chars.
+for (var i = 1; i < 128; i++) {
+  assertEquals(alpha[i], alphaStr.charAt(i));
+  assertEquals(String.fromCharCode(i), alphaStr.charAt(i));
+}