Avoid sign issues with the fast case code for Math.floor().

src/math.js

 // Copyright 2006-2008 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 77 matching lines @@
 
 // ECMA 262 - 15.8.2.8
 function MathExp(x) {
   if (!IS_NUMBER(x)) x = ToNumber(x);
   return %Math_exp(x);
 }
 
 // ECMA 262 - 15.8.2.9
 function MathFloor(x) {
   if (!IS_NUMBER(x)) x = ToNumber(x);
-  if (0 < x && x <= 0xFFFFFFFF) {
-    // Numbers in the range [0, 2^32) can be floored by converting
+  if (0 < x && x <= 0x7FFFFFFF) {
+    // Numbers in the range [0, 2^31) can be floored by converting
     // them to an unsigned 32-bit value using the shift operator.
     // We avoid doing so for -0, because the result of Math.floor(-0)
     // has to be -0, which wouldn't be the case with the shift.
     return x << 0;
   } else {
     return %Math_floor(x);
   }
 }
 
 // ECMA 262 - 15.8.2.10
@@ skipping 123 matching lines @@
     "tan", MathTan,
     "atan2", MathAtan2,
     "pow", MathPow,
     "max", MathMax,
     "min", MathMin
   ));
 };
 
 
 SetupMath();