Make uncaught errors terminating isolates only get printed if nobody is listening.

runtime/vm/isolate.h

 // Copyright (c) 2013, 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.
 
 #ifndef VM_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 459 matching lines @@
   // list of pause events.
   bool AddResumeCapability(const Capability& capability);
   bool RemoveResumeCapability(const Capability& capability);
 
   void AddExitListener(const SendPort& listener, const Instance& response);
   void RemoveExitListener(const SendPort& listener);
   void NotifyExitListeners();
 
   void AddErrorListener(const SendPort& listener);
   void RemoveErrorListener(const SendPort& listener);
-  void NotifyErrorListeners(const String& msg, const String& stacktrace);
+  bool NotifyErrorListeners(const String& msg, const String& stacktrace);
 
   bool ErrorsFatal() const { return errors_fatal_; }
   void SetErrorsFatal(bool val) { errors_fatal_ = val; }
 
   Random* random() { return &random_; }
 
   Simulator* simulator() const { return simulator_; }
   void set_simulator(Simulator* value) { simulator_ = value; }
 
   Dart_GcPrologueCallback gc_prologue_callback() const {
@@ skipping 599 matching lines @@
   uint8_t* serialized_message_;
   intptr_t serialized_message_len_;
   Isolate::Flags isolate_flags_;
   bool paused_;
   bool errors_are_fatal_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_