Migrate most uses of StackZone(Isolate*) to Thread*.

runtime/vm/zone_test.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 "platform/assert.h"
 #include "vm/dart.h"
 #include "vm/isolate.h"
 #include "vm/unit_test.h"
 #include "vm/zone.h"
 
@@ skipping 148 matching lines @@
     second->slot = 87;
     EXPECT_EQ(42, first->slot);
     EXPECT_EQ(87, second->slot);
   }
   EXPECT(isolate->current_zone() == NULL);
   Dart_ShutdownIsolate();
 }
 
 
 TEST_CASE(PrintToString) {
-  StackZone zone(Isolate::Current());
+  StackZone zone(Thread::Current());
   const char* result = zone.GetZone()->PrintToString("Hello %s!", "World");
   EXPECT_STREQ("Hello World!", result);
 }
 
 }  // namespace dart