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

runtime/vm/flow_graph_builder.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_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/class_finalizer.h"
@@ skipping 1334 matching lines @@
   }
   EffectGraphVisitor::VisitBinaryOpNode(node);
 }
 
 
 static const String& BinaryOpAndMaskName(BinaryOpNode* node) {
   if (node->kind() == Token::kSHL) {
     return Library::PrivateCoreLibName(Symbols::_leftShiftWithMask32());
   }
   UNIMPLEMENTED();
-  return String::ZoneHandle(Isolate::Current(), String::null());
+  return String::ZoneHandle(Thread::Current()->zone(), String::null());
 }
 
 
 // <Expression> :: BinaryOp { kind:  Token::Kind
 //                            left:  <Expression>
 //                            right: <Expression>
 //                            mask32: constant }
 void EffectGraphVisitor::VisitBinaryOpWithMask32Node(
     BinaryOpWithMask32Node* node) {
   ASSERT((node->kind() != Token::kAND) && (node->kind() != Token::kOR));
@@ skipping 3002 matching lines @@
   Report::MessageF(Report::kBailout,
                    Script::Handle(function.script()),
                    function.token_pos(),
                    "FlowGraphBuilder Bailout: %s %s",
                    String::Handle(function.name()).ToCString(),
                    reason);
   UNREACHABLE();
 }
 
 }  // namespace dart