Allows turning on checked mode on a per-isolate basis

runtime/lib/object.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/bootstrap_natives.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/code_patcher.h"
 #include "vm/exceptions.h"
 #include "vm/heap.h"
@@ skipping 241 matching lines @@
     }
     String& bound_error_message =  String::Handle();
     if (bound_error.IsNull()) {
       const String& dst_name = String::ZoneHandle(
           Symbols::New(Exceptions::kCastErrorDstName));
 
       Exceptions::CreateAndThrowTypeError(
           location, instance_type_name, type_name,
           dst_name, Object::null_string());
     } else {
-      ASSERT(FLAG_enable_type_checks);
+      ASSERT(Isolate::Current()->TypeChecksEnabled());
       bound_error_message = String::New(bound_error.ToErrorCString());
       Exceptions::CreateAndThrowTypeError(
           location, instance_type_name, Symbols::Empty(),
           Symbols::Empty(), bound_error_message);
     }
     UNREACHABLE();
   }
   return instance.raw();
 }
 
@@ skipping 27 matching lines @@
   // Currently all errors are Dart instances, e.g. I/O errors
   // created by deferred loading code. LanguageErrors from
   // failed loading or finalization attempts are propagated and result
   // in the isolate's death.
   const Instance& error = Instance::Handle(prefix.LoadError());
   return error.raw();
 }
 
 
 }  // namespace dart