[es6] Handle conflicts for sloppy let

src/runtime/runtime-scopes.cc

Index: src/runtime/runtime-scopes.cc
diff --git a/src/runtime/runtime-scopes.cc b/src/runtime/runtime-scopes.cc
index 953a09f16df7127c2654207571fe61cb483f3c59..b4506512a188e92ee9a02e341582a7fbe1ce30f6 100644
--- a/src/runtime/runtime-scopes.cc
+++ b/src/runtime/runtime-scopes.cc
@@ -1026,6 +1026,12 @@ RUNTIME_FUNCTION(Runtime_StoreLookupSlot) {
 
   // The property was found in a context slot.
   if (index >= 0) {
+    if ((binding_flags == MUTABLE_CHECK_INITIALIZED ||
+         binding_flags == IMMUTABLE_CHECK_INITIALIZED_HARMONY) &&
+        Handle<Context>::cast(holder)->is_the_hole(index)) {
+      THROW_NEW_ERROR_RETURN_FAILURE(
+          isolate, NewReferenceError(MessageTemplate::kNotDefined, name));
+    }
     if ((attributes & READ_ONLY) == 0) {
       Handle<Context>::cast(holder)->set(index, *value);
     } else if (is_strict(language_mode)) {