Implemented one-char cache lookup in generated code.

test/mjsunit/string-index.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 134 matching lines @@
 assertEquals('T', s[0]);
 assertEquals('t', s2);
 
 var S2 = (S[-2] = 'o');
 assertEquals('o', S[-2]);
 assertEquals('o', S2);
 
 var s2 = (s[-2] = 't');
 assertEquals('undefined', typeof(s[-2]));
 assertEquals('t', s2);
+
+// 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[0];
+}
+var alphaStr = alpha.join("");
+
+// Now test chars.
+for (var i = 1; i < 128; i++) {
+  assertEquals(alpha[i], alphaStr[i]);
+  assertEquals(String.fromCharCode(i), alphaStr[i]);
+}