Introduce NewScriptScope that creates a top-level scope

It's the only Scope type that has outer scope nullptr; and it always has outer scope nullptr.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2158913005
Cr-Commit-Position: refs/heads/master@{#37907}

src/parsing/parser-base.h

@@ -615,9 +615,16 @@ class ParserBase : public Traits {
     Mode old_mode_;
   };
 
+  Scope* NewScriptScope() {
+    return new (zone()) Scope(zone(), nullptr, SCRIPT_SCOPE, kNormalFunction);
+  }
+
   Scope* NewScope(Scope* parent, ScopeType scope_type) {
-    // Must always pass the function kind for FUNCTION_SCOPE.
-    DCHECK(scope_type != FUNCTION_SCOPE);
+    // Must always use the specific constructors for the blacklisted scope
+    // types.
+    DCHECK_NE(FUNCTION_SCOPE, scope_type);
+    DCHECK_NE(SCRIPT_SCOPE, scope_type);
+    DCHECK_NOT_NULL(parent);
     Scope* result =
         new (zone()) Scope(zone(), parent, scope_type, kNormalFunction);
     if (scope_type == MODULE_SCOPE) result->DeclareThis(ast_value_factory());

src/parsing/parser.cc

@@ -939,7 +939,7 @@ FunctionLiteral* Parser::DoParseProgram(ParseInfo* info) {
   {
     // TODO(wingo): Add an outer SCRIPT_SCOPE corresponding to the native
     // context, which will have the "this" binding for script scopes.
-    Scope* scope = NewScope(nullptr, SCRIPT_SCOPE);
+    Scope* scope = NewScriptScope();
     info->set_script_scope(scope);
     if (!info->context().is_null() && !info->context()->IsNativeContext()) {
       scope = Scope::DeserializeScopeChain(info->isolate(), zone(),
@@ -1103,7 +1103,7 @@ FunctionLiteral* Parser::ParseLazy(Isolate* isolate, ParseInfo* info,
 
   {
     // Parse the function literal.
-    Scope* scope = NewScope(nullptr, SCRIPT_SCOPE);
+    Scope* scope = NewScriptScope();
     info->set_script_scope(scope);
     if (!info->context().is_null()) {
       // Ok to use Isolate here, since lazy function parsing is only done in the

src/parsing/preparser.cc

@@ -131,7 +131,7 @@ PreParser::PreParseResult PreParser::PreParseLazyFunction(
   use_counts_ = use_counts;
   // Lazy functions always have trivial outer scopes (no with/catch scopes).
   DCHECK_NULL(scope_state_);
-  Scope* top_scope = NewScope(nullptr, SCRIPT_SCOPE);
+  Scope* top_scope = NewScriptScope();
   PreParserFactory top_factory(nullptr);
   FunctionState top_state(&function_state_, &scope_state_, top_scope,
                           kNormalFunction, &top_factory);

src/parsing/preparser.h

@@ -1040,7 +1040,7 @@ class PreParser : public ParserBase<PreParserTraits> {
   PreParseResult PreParseProgram(int* materialized_literals = 0,
                                  bool is_module = false) {
     DCHECK_NULL(scope_state_);
-    Scope* scope = NewScope(nullptr, SCRIPT_SCOPE);
+    Scope* scope = NewScriptScope();
 
     // ModuleDeclarationInstantiation for Source Text Module Records creates a
     // new Module Environment Record whose outer lexical environment record is