Reapply "Improve hashCode for closure objects" with fixes.

runtime/vm/isolate_reload.cc

Index: runtime/vm/isolate_reload.cc
diff --git a/runtime/vm/isolate_reload.cc b/runtime/vm/isolate_reload.cc
index 5537b5adc9e0e7288d1bf456c9859f8ef52a10fa..eeff480c8cbcb0fc97031550570608a9dc312f35 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -357,7 +357,11 @@ class BecomeMapTraits {
     } else if (obj.IsField()) {
       return String::HashRawSymbol(Field::Cast(obj).name());
     } else if (obj.IsInstance()) {
-      return Smi::Handle(Smi::RawCast(Instance::Cast(obj).HashCode())).Value();
+      Object& hashObj = Object::Handle(Instance::Cast(obj).HashCode());
+      if (hashObj.IsError()) {
+        Exceptions::PropagateError(Error::Cast(hashObj));
+      }
+      return Smi::Cast(hashObj).Value();
     }
     return 0;
   }