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

runtime/vm/native_entry_test.cc

 // Copyright (c) 2011, 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/native_entry_test.h"
 
 #include "vm/assembler.h"
 #include "vm/code_patcher.h"
 #include "vm/dart_api_impl.h"
 #include "vm/native_entry.h"
@@ skipping 38 matching lines @@
     result += arg_value;
   }
   Dart_SetReturnValue(args, Dart_NewInteger(result));
 }
 
 
 // Test for accepting null arguments in native function.
 // Arg0-4: 5 smis or null.
 // Result: a smi representing the sum of all non-null arguments.
 void TestNonNullSmiSum(Dart_NativeArguments args) {
-  Isolate* isolate = Isolate::Current();
+  Isolate* isolate = Isolate::Current();  // Used by GET_NATIVE_ARGUMENT.
   int64_t result = 0;
   int arg_count = Dart_GetNativeArgumentCount(args);
   // Test the lower level macro GET_NATIVE_ARGUMENT.
   NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
+  Zone* zone = Thread::Current()->zone();  // Used by GET_NATIVE_ARGUMENT.
   for (int i = 0; i < arg_count; i++) {
     Dart_Handle arg = Dart_GetNativeArgument(args, i);
     GET_NATIVE_ARGUMENT(Integer, argument, arguments->NativeArgAt(i));
     EXPECT(argument.IsInteger());  // May be null.
     EXPECT_EQ(Api::UnwrapHandle(arg), argument.raw());  // May be null.
     int64_t arg_value = -1;
     if (argument.IsNull()) {
       EXPECT_ERROR(Dart_IntegerToInt64(arg, &arg_value),
                    "Dart_IntegerToInt64 expects argument 'integer' "
                    "to be non-null.");
     } else {
       EXPECT_VALID(Dart_IntegerToInt64(arg, &arg_value));
       EXPECT_EQ(arg_value, argument.AsInt64Value());
       // Ignoring overflow in the addition below.
       result += arg_value;
     }
   }
   Dart_SetReturnValue(args, Dart_NewInteger(result));
 }
 
 }  // namespace dart