Provide stdout and stderr output in the Observatory debugger.

runtime/vm/service_isolate.cc

 // Copyright (c) 2015, 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/service_isolate.h"
 
 #include "vm/compiler.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/isolate.h"
@@ skipping 286 matching lines @@
     }
   }
 
   static void ListenStream(Dart_NativeArguments args) {
     NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
     Isolate* isolate = arguments->thread()->isolate();
     StackZone stack_zone(isolate);
     Zone* zone = stack_zone.GetZone();  // Used by GET_NON_NULL_NATIVE_ARGUMENT.
     HANDLESCOPE(isolate);
     GET_NON_NULL_NATIVE_ARGUMENT(String, stream_id, arguments->NativeArgAt(0));
-    Service::ListenStream(stream_id.ToCString());
+    bool result = Service::ListenStream(stream_id.ToCString());
+    arguments->SetReturn(Bool::Get(result));
   }
 
   static void CancelStream(Dart_NativeArguments args) {
     NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
     Isolate* isolate = arguments->thread()->isolate();
     StackZone stack_zone(isolate);
     Zone* zone = stack_zone.GetZone();  // Used by GET_NON_NULL_NATIVE_ARGUMENT.
     HANDLESCOPE(isolate);
     GET_NON_NULL_NATIVE_ARGUMENT(String, stream_id, arguments->NativeArgAt(0));
     Service::CancelStream(stream_id.ToCString());
@@ skipping 488 matching lines @@
     return result;
   }
   Dart_Handle source = GetSource(url_string);
   if (Dart_IsError(source)) {
     return source;
   }
   return Dart_LoadSource(library, url, source, 0, 0);
 }
 
 }  // namespace dart