[es2015] Precompute the descriptive string for symbols.

src/objects-debug.cc

 // Copyright 2012 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/objects.h"
 
 #include "src/assembler-inl.h"
 #include "src/bootstrapper.h"
 #include "src/disasm.h"
 #include "src/disassembler.h"
@@ skipping 257 matching lines @@
   HeapObject* ho = HeapObject::cast(p);
   CHECK(ho->GetHeap()->Contains(ho));
 }
 
 
 void Symbol::SymbolVerify() {
   CHECK(IsSymbol());
   CHECK(HasHashCode());
   CHECK(Hash() > 0u);
   CHECK(name()->IsUndefined(GetIsolate()) || name()->IsString());
+  CHECK(descriptive_string()->IsString());
 }
 
 
 void HeapNumber::HeapNumberVerify() {
   CHECK(IsHeapNumber() || IsMutableHeapNumber());
 }
 
 void ByteArray::ByteArrayVerify() {
   CHECK(IsByteArray());
 }
@@ skipping 1360 matching lines @@
 
   // Both are done at the same time.
   CHECK_EQ(new_it.done(), old_it.done());
 }
 
 
 #endif  // DEBUG
 
 }  // namespace internal
 }  // namespace v8