Fix missing cases of empty_string canonicalization

src/factory.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/factory.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
 #include "src/ast/ast.h"
 #include "src/base/bits.h"
@@ skipping 802 matching lines @@
   return slice;
 }
 
 
 MaybeHandle<String> Factory::NewExternalStringFromOneByte(
     const ExternalOneByteString::Resource* resource) {
   size_t length = resource->length();
   if (length > static_cast<size_t>(String::kMaxLength)) {
     THROW_NEW_ERROR(isolate(), NewInvalidStringLengthError(), String);
   }
+  if (length == 0) return empty_string();
 
   Handle<Map> map;
   if (resource->IsCompressible()) {
     // TODO(hajimehoshi): Rename this to 'uncached_external_one_byte_string_map'
     map = short_external_one_byte_string_map();
   } else {
     map = external_one_byte_string_map();
   }
   Handle<ExternalOneByteString> external_string =
       New<ExternalOneByteString>(map, NEW_SPACE);
   external_string->set_length(static_cast<int>(length));
   external_string->set_hash_field(String::kEmptyHashField);
   external_string->set_resource(resource);
 
   return external_string;
 }
 
 
 MaybeHandle<String> Factory::NewExternalStringFromTwoByte(
     const ExternalTwoByteString::Resource* resource) {
   size_t length = resource->length();
   if (length > static_cast<size_t>(String::kMaxLength)) {
     THROW_NEW_ERROR(isolate(), NewInvalidStringLengthError(), String);
   }
+  if (length == 0) return empty_string();
 
   // For small strings we check whether the resource contains only
   // one byte characters.  If yes, we use a different string map.
   static const size_t kOneByteCheckLengthLimit = 32;
   bool is_one_byte = length <= kOneByteCheckLengthLimit &&
       String::IsOneByte(resource->data(), static_cast<int>(length));
   Handle<Map> map;
   if (resource->IsCompressible()) {
     // TODO(hajimehoshi): Rename these to 'uncached_external_string_...'.
     map = is_one_byte ? short_external_string_with_one_byte_data_map()
@@ skipping 1989 matching lines @@
     Handle<AccessorInfo> prototype =
         Accessors::FunctionPrototypeInfo(isolate(), rw_attribs);
     Descriptor d = Descriptor::AccessorConstant(
         Handle<Name>(Name::cast(prototype->name())), prototype, rw_attribs);
     map->AppendDescriptor(&d);
   }
 }
 
 }  // namespace internal
 }  // namespace v8