Do not allow a hot reload if the isolate has a sticky error

runtime/vm/service.cc

 // 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.
 
 #include "vm/service.h"
 
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
 #include "platform/globals.h"
 
@@ skipping 2502 matching lines @@
     js->PrintError(kFeatureDisabled,
         "Cannot reload source when running an app snapshot.");
     return true;
   }
   Dart_LibraryTagHandler handler = isolate->library_tag_handler();
   if (handler == NULL) {
     js->PrintError(kFeatureDisabled,
                    "A library tag handler must be installed.");
     return true;
   }
+  if ((isolate->sticky_error() != Error::null()) ||
+      (Thread::Current()->sticky_error() != Error::null())) {
+    js->PrintError(kIsolateReloadBarred,
+                   "This isolate cannot reload sources anymore because there "
+                   "was an unhandled exception error. Restart the isolate.");
+    return true;
+  }
   if (isolate->IsReloading()) {
     js->PrintError(kIsolateIsReloading,
                    "This isolate is being reloaded.");
     return true;
   }
   if (!isolate->CanReload()) {
     js->PrintError(kFeatureDisabled,
                    "This isolate cannot reload sources right now.");
     return true;
   }
@@ skipping 1631 matching lines @@
     if (strcmp(method_name, method.name) == 0) {
       return &method;
     }
   }
   return NULL;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart