Unify handling of identical(NaN, NaN) between generated code and Dart API.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 37108)
+++ runtime/vm/object.cc	(working copy)
@@ -13048,13 +13048,7 @@
     return Integer::Cast(*this).Equals(other);
   }
   if (IsDouble() && other.IsDouble()) {
-    if (Double::Cast(*this).CanonicalizeEquals(other)) return true;
-    // Check for NaN.
-    const Double& a_double = Double::Cast(*this);
-    const Double& b_double = Double::Cast(other);
-    if (isnan(a_double.value()) && isnan(b_double.value())) {
-      return true;
-    }
+    return Double::Cast(*this).CanonicalizeEquals(other);
   }
   return false;
 }