Add sparse array handling to Array.protoype.indexOf/lastIndexOf.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 1122d331a4fd0519799fc6bf2b97ace49889109c..7cfd5a659bea55e869467b5e5c7101f196368138 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -8199,8 +8199,10 @@ static Object* Runtime_SwapElements(Arguments args) {
 
 
 // Returns an array that tells you where in the [0, length) interval an array
-// might have elements.  Can either return keys or intervals.  Keys can have
-// gaps in (undefined).  Intervals can also span over some undefined keys.
+// might have elements.  Can either return keys (positive integers) or
+// intervals (pair of a negative integer (-start-1) followed by a
+// positive (length)) or undefined values.
+// Intervals can span over some keys that are not in the object.
 static Object* Runtime_GetArrayKeys(Arguments args) {
   ASSERT(args.length() == 2);
   HandleScope scope;