[es2015] Precompute the descriptive string for symbols.

src/builtins/builtins-symbol-gen.cc

 // Copyright 2017 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/builtins/builtins-utils-gen.h"
 #include "src/builtins/builtins.h"
 #include "src/code-stub-assembler.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 11 matching lines @@
   Return(result);
 }
 
 // ES6 #sec-symbol.prototype.tostring
 TF_BUILTIN(SymbolPrototypeToString, CodeStubAssembler) {
   Node* context = Parameter(Descriptor::kContext);
   Node* receiver = Parameter(Descriptor::kReceiver);
 
   Node* value = ToThisValue(context, receiver, PrimitiveType::kSymbol,
                             "Symbol.prototype.toString");
-  Node* result = CallRuntime(Runtime::kSymbolDescriptiveString, context, value);
+  Node* result = LoadObjectField(value, Symbol::kDescriptiveStringOffset);
   Return(result);
 }
 
 // ES6 #sec-symbol.prototype.valueof
 TF_BUILTIN(SymbolPrototypeValueOf, CodeStubAssembler) {
   Node* context = Parameter(Descriptor::kContext);
   Node* receiver = Parameter(Descriptor::kReceiver);
 
   Node* result = ToThisValue(context, receiver, PrimitiveType::kSymbol,
                              "Symbol.prototype.valueOf");
   Return(result);
 }
 
 }  // namespace internal
 }  // namespace v8