Address some leftover review comments from previous CL.

vm/object.cc

Index: vm/object.cc
===================================================================
--- vm/object.cc	(revision 9718)
+++ vm/object.cc	(working copy)
@@ -7830,7 +7830,7 @@
   ASSERT(str.IsOneByteString());
   const OneByteString& onestr = OneByteString::Cast(str);
   int64_t value;
-  if (!OS::StringToInteger(onestr.ToCString(), &value)) {
+  if (!OS::StringToInt64(onestr.ToCString(), &value)) {
     const Bigint& big = Bigint::Handle(Bigint::New(onestr, space));
     ASSERT(!BigintOperations::FitsIntoSmi(big));
     ASSERT(!BigintOperations::FitsIntoMint(big));