[regexp] Properly flatten string during initialization

src/runtime/runtime-test.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/runtime/runtime-utils.h"
 
 #include <memory>
 
 #include "src/arguments.h"
 #include "src/assembler-inl.h"
@@ skipping 53 matching lines @@
 
 RUNTIME_FUNCTION(Runtime_ConstructDouble) {
   HandleScope scope(isolate);
   DCHECK_EQ(2, args.length());
   CONVERT_NUMBER_CHECKED(uint32_t, hi, Uint32, args[0]);
   CONVERT_NUMBER_CHECKED(uint32_t, lo, Uint32, args[1]);
   uint64_t result = (static_cast<uint64_t>(hi) << 32) | lo;
   return *isolate->factory()->NewNumber(uint64_to_double(result));
 }
 
+RUNTIME_FUNCTION(Runtime_ConstructConsString) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(2, args.length());
+  CONVERT_ARG_HANDLE_CHECKED(String, left, 0);
+  CONVERT_ARG_HANDLE_CHECKED(String, right, 1);
+
+  CHECK(left->IsOneByteRepresentation());
+  CHECK(right->IsOneByteRepresentation());
+
+  const bool kIsOneByte = true;
+  const int length = left->length() + right->length();
+  return *isolate->factory()->NewConsString(left, right, length, kIsOneByte);
+}
+
 RUNTIME_FUNCTION(Runtime_DeoptimizeFunction) {
   HandleScope scope(isolate);
   DCHECK_EQ(1, args.length());
 
   // This function is used by fuzzers to get coverage in compiler.
   // Ignore calls on non-function objects to avoid runtime errors.
   CONVERT_ARG_HANDLE_CHECKED(Object, function_object, 0);
   if (!function_object->IsJSFunction()) {
     return isolate->heap()->undefined_value();
   }
@@ skipping 833 matching lines @@
     CHECK(HeapObject::cast(*object)->map()->IsMap());
   } else {
     CHECK(object->IsSmi());
   }
 #endif
   return isolate->heap()->ToBoolean(true);
 }
 
 }  // namespace internal
 }  // namespace v8