[ast] Add DCHECKs that ensure the AST contains no dead scopes.

Until recently, it sometimes did.

BUG=

Change-Id: I8785c1865909e6f46693b71c9146d4fa17114fce
Reviewed-on: https://chromium-review.googlesource.com/444188
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43291}

src/ast/ast-numbering.cc

@@ -281,9 +281,10 @@ void AstNumberingVisitor::VisitBlock(Block* node) {
   IncrementNodeCount();
   node->set_base_id(ReserveIdRange(Block::num_ids()));
   Scope* scope = node->scope();
+  DCHECK(scope == nullptr || !scope->HasBeenRemoved());
   // TODO(ishell): remove scope->NeedsContext() condition once v8:5927 is fixed.
   // Current logic mimics what BytecodeGenerator::VisitBlock() does.
-  if (scope != NULL && scope->NeedsContext()) {
+  if (scope != nullptr && scope->NeedsContext()) {
     LanguageModeScope language_mode_scope(this, scope->language_mode());
     VisitStatementsAndDeclarations(node);
   } else {
@@ -293,6 +294,7 @@ void AstNumberingVisitor::VisitBlock(Block* node) {
 
 void AstNumberingVisitor::VisitStatementsAndDeclarations(Block* node) {
   Scope* scope = node->scope();
+  DCHECK(scope == nullptr || !scope->HasBeenRemoved());
   if (scope) VisitDeclarations(scope->declarations());
   VisitStatements(node->statements());
 }
@@ -363,6 +365,7 @@ void AstNumberingVisitor::VisitWhileStatement(WhileStatement* node) {
 
 
 void AstNumberingVisitor::VisitTryCatchStatement(TryCatchStatement* node) {
+  DCHECK(node->scope() == nullptr || !node->scope()->HasBeenRemoved());
   IncrementNodeCount();
   DisableFullCodegenAndCrankshaft(kTryCatchStatement);
   {
@@ -662,6 +665,8 @@ void AstNumberingVisitor::VisitRewritableExpression(
 
 bool AstNumberingVisitor::Renumber(FunctionLiteral* node) {
   DeclarationScope* scope = node->scope();
+  DCHECK(!scope->HasBeenRemoved());
+
   if (scope->new_target_var() != nullptr ||
       scope->this_function_var() != nullptr) {
     DisableFullCodegenAndCrankshaft(kSuperReference);