Only propagate reload errors when there are Dart frames on the stack.

runtime/vm/isolate_reload.cc

 // Copyright (c) 2016, 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/isolate_reload.h"
 
 #include "vm/become.h"
 #include "vm/bit_vector.h"
 #include "vm/code_generator.h"
 #include "vm/compiler.h"
@@ skipping 616 matching lines @@
 
     Dart_Handle retval = (I->library_tag_handler())(
         Dart_kScriptTag, Api::NewHandle(thread, packages_url.raw()),
         Api::NewHandle(thread, root_lib_url.raw()));
     result = Api::UnwrapHandle(retval);
   }
   //
   // WEIRD CONTROL FLOW ENDS.
   TIR_Print("---- EXITED TAG HANDLER\n");
 
   BackgroundCompiler::Enable();

[siva, 2017/03/16 18:09:45]

We should probably enable the background compiler as the last step after all the
checks are done.

 
-  if (result.IsUnwindError() || result.IsUnhandledException()) {
-    // If the tag handler returns with an UnwindError or an UnhandledException
-    // error, propagate it and give up.
+  if (result.IsUnwindError()) {
+    // We can only propagate errors when there are Dart frames on the stack.
+    // TODO(johnmccutchan): Fix dartbug.com/29092.
+    if (thread->top_exit_frame_info() == 0) {
+      FATAL(
+          "Got an Unwind Error in the middle of a reload. "
+          "http://dartbug.com/29092");
+    }
+    // If the tag handler returns with an UnwindError error, propagate it and
+    // give up.
     Exceptions::PropagateError(Error::Cast(result));
     UNREACHABLE();
   }
 
   // Other errors (e.g. a parse error) are captured by the reload system.
   if (result.IsError()) {
     FinalizeFailedLoad(Error::Cast(result));
   }
 }
 
@@ skipping 1234 matching lines @@
         ASSERT(!super_cls.IsNull());
         super_cls.AddDirectSubclass(cls);
       }
     }
   }
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart