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

runtime/vm/flow_graph.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph.h"
 
 #include "vm/bit_vector.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/il_printer.h"
 #include "vm/intermediate_language.h"
@@ skipping 82 matching lines @@
       ? &optimized_block_order_
       : &reverse_postorder_;
 }
 
 
 ConstantInstr* FlowGraph::GetConstant(const Object& object) {
   ConstantInstr* constant = constant_instr_pool_.Lookup(object);
   if (constant == NULL) {
     // Otherwise, allocate and add it to the pool.
     constant = new(zone()) ConstantInstr(
-        Object::ZoneHandle(isolate(), object.raw()));
+        Object::ZoneHandle(zone(), object.raw()));
     constant->set_ssa_temp_index(alloc_ssa_temp_index());
 
     AddToInitialDefinitions(constant);
     constant_instr_pool_.Insert(constant);
   }
   return constant;
 }
 
 
 void FlowGraph::AddToInitialDefinitions(Definition* defn) {
@@ skipping 1274 matching lines @@
 }
 
 
 bool BlockEffects::IsSideEffectFreePath(BlockEntryInstr* from,
                                         BlockEntryInstr* to) const {
   return available_at_[to->postorder_number()]->Contains(
       from->postorder_number());
 }
 
 }  // namespace dart