PreParsing inner funcs: Do fewer variable name lookups.

It's unnecessarily to first lookup and then insert.

BUG=v8:5501

Review-Url: https://codereview.chromium.org/2608333003
Cr-Commit-Position: refs/heads/master@{#42065}

src/ast/scopes.cc

@@ -1037,11 +1037,10 @@ void Scope::DeclareVariableName(const AstRawString* name, VariableMode mode) {
   DCHECK(!is_eval_scope());
   DCHECK(is_declaration_scope() ||
          (IsLexicalVariableMode(mode) && is_block_scope()));
+  DCHECK(scope_info_.is_null());
 
   // Declare the variable in the declaration scope.
-  if (LookupLocal(name) == nullptr) {
   variables_.DeclareName(zone(), name);
-  }
 }
 
 VariableProxy* Scope::NewUnresolved(AstNodeFactory* factory,

src/ast/scopes.h

@@ -34,8 +34,8 @@ class VariableMap: public ZoneHashMap {
                     MaybeAssignedFlag maybe_assigned_flag = kNotAssigned,
                     bool* added = nullptr);
 
-  // Records that "name" exists but doesn't create a Variable. Useful for
-  // preparsing.
+  // Records that "name" exists (if not recorded yet) but doesn't create a
+  // Variable. Useful for preparsing.
   void DeclareName(Zone* zone, const AstRawString* name);
 
   Variable* Lookup(const AstRawString* name);