Make root-zone handleUncaughtError rethrow with the correct stack.

runtime/lib/errors.cc

Index: runtime/lib/errors.cc
diff --git a/runtime/lib/errors.cc b/runtime/lib/errors.cc
index 8383a307634ed1be18f7ce59591253d79ff75652..31c3a85cbbd3f9b58cca34a2f944313832ab7d41 100644
--- a/runtime/lib/errors.cc
+++ b/runtime/lib/errors.cc
@@ -126,4 +126,12 @@ DEFINE_NATIVE_ENTRY(AbstractClassInstantiationError_throwNew, 2) {
   return Object::null();
 }
 
+// Rethrow an error with a stacktrace.
+DEFINE_NATIVE_ENTRY(Zone_rethrow, 2) {
+  GET_NON_NULL_NATIVE_ARGUMENT(Instance, error, arguments->NativeArgAt(0));
+  GET_NON_NULL_NATIVE_ARGUMENT(Instance, stacktrace, arguments->NativeArgAt(1));
+  Exceptions::ReThrow(thread, error, stacktrace);
+  return Object::null();
+}
+
 }  // namespace dart