Cleanups: addressed your comments from https://codereview.chromium.org/14962008/

runtime/lib/double.cc

Index: runtime/lib/double.cc
===================================================================
--- runtime/lib/double.cc	(revision 22880)
+++ runtime/lib/double.cc	(working copy)
@@ -80,8 +80,8 @@
     Exceptions::ThrowByType(Exceptions::kUnsupported, args);
   }
   const Bigint& big = Bigint::Handle(BigintOperations::NewFromDouble(val));
-  if (BigintOperations::FitsIntoMint(big)) {
-    return Integer::New(BigintOperations::ToMint(big));
+  if (BigintOperations::FitsIntoInt64(big)) {
+    return Integer::New(BigintOperations::ToInt64(big));
   } else {
     return big.raw();
   }