[es6] Handle conflicts for sloppy let

src/runtime/runtime-scopes.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/arguments.h"
 #include "src/frames-inl.h"
 #include "src/messages.h"
@@ skipping 1008 matching lines @@
   BindingFlags binding_flags;
   Handle<Object> holder =
       context->Lookup(name, flags, &index, &attributes, &binding_flags);
   if (holder.is_null()) {
     // In case of JSProxy, an exception might have been thrown.
     if (isolate->has_pending_exception()) return isolate->heap()->exception();
   }
 
   // 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)) {
       // Setting read only property in strict mode.
       THROW_NEW_ERROR_RETURN_FAILURE(
           isolate, NewTypeError(MessageTemplate::kStrictCannotAssign, name));
     }
     return *value;
   }
 
@@ skipping 105 matching lines @@
   return Smi::FromInt(frame->GetArgumentsLength());
 }
 
 
 RUNTIME_FUNCTION(Runtime_Arguments) {
   SealHandleScope shs(isolate);
   return __RT_impl_Runtime_GetArgumentsProperty(args, isolate);
 }
 }  // namespace internal
 }  // namespace v8