Keep zone cached in SnapshotReader to allow removing ZoneHandle(Isolate*) interface.

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
===================================================================
--- runtime/vm/dart.cc	(revision 44579)
+++ runtime/vm/dart.cc	(working copy)
@@ -238,7 +238,7 @@
       OS::Print("Size of isolate snapshot = %" Pd "\n", snapshot->length());
     }
     SnapshotReader reader(snapshot->content(), snapshot->length(),
-                          Snapshot::kFull, isolate);
+                          Snapshot::kFull, isolate, zone.GetZone());
     const Error& error = Error::Handle(reader.ReadFullSnapshot());
     if (!error.IsNull()) {
       return error.raw();