Fix missing cases of empty_string canonicalization

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/bootstrapper.h"
 #include "src/disasm.h"
 #include "src/disassembler.h"
 #include "src/field-type.h"
@@ skipping 534 matching lines @@
   VerifyObjectField(kEndPositionOffset);
   VerifyObjectField(kArgumentsOffset);
   VerifyObjectField(kScriptOffset);
   VerifyObjectField(kStackFramesOffset);
 }
 
 
 void String::StringVerify() {
   CHECK(IsString());
   CHECK(length() >= 0 && length() <= Smi::kMaxValue);
+  CHECK_IMPLIES(length() == 0, this == GetHeap()->empty_string());
   if (IsInternalizedString()) {
     CHECK(!GetHeap()->InNewSpace(this));
   }
   if (IsConsString()) {
     ConsString::cast(this)->ConsStringVerify();
   } else if (IsSlicedString()) {
     SlicedString::cast(this)->SlicedStringVerify();
   } else if (IsThinString()) {
     ThinString::cast(this)->ThinStringVerify();
   }
@@ skipping 996 matching lines @@
 
   // Both are done at the same time.
   CHECK_EQ(new_it.done(), old_it.done());
 }
 
 
 #endif  // DEBUG
 
 }  // namespace internal
 }  // namespace v8