Trigger debug event on not yet caught exception in promises.

src/promise.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 174 matching lines @@
 function PromiseHandle(value, handler, deferred) {
   try {
     var result = handler(value);
     if (result === deferred.promise)
       throw MakeTypeError('promise_cyclic', [result]);
     else if (IsPromise(result))
       %_CallFunction(result, deferred.resolve, deferred.reject, PromiseChain);
     else
       deferred.resolve(result);
   } catch(e) {
-    // TODO(rossberg): perhaps log uncaught exceptions below.
-    try { deferred.reject(e) } catch(e) {}
+    if (GET_PRIVATE(deferred.promise, promiseOnReject).length == 0) {
+      // There is no reject handler defined (yet).  The debugger should know.
+      %DebugPendingExceptionInPromise(e, deferred.promise);
+    }
+    try { deferred.reject(e) } catch(e) {
+      // Reject handler threw (must have been a custom one).
+      %DebugPendingExceptionInPromise(e, deferred.promise);
+    }
   }
 }
 
 
 // Multi-unwrapped chaining with thenable coercion.
 
 function PromiseThen(onResolve, onReject) {
   onResolve =
     IS_NULL_OR_UNDEFINED(onResolve) ? PromiseIdResolveHandler : onResolve;
   onReject =
@@ skipping 111 matching lines @@
     "resolve", PromiseCast
   ]);
   InstallFunctions($Promise.prototype, DONT_ENUM, [
     "chain", PromiseChain,
     "then", PromiseThen,
     "catch", PromiseCatch
   ]);
 }
 
 SetUpPromise();