Reland of [parsing] Fix maybe-assigned for loop variables.

src/parsing/preparser.cc

 // Copyright 2011 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 <cmath>
 
 #include "src/allocation.h"
 #include "src/base/logging.h"
 #include "src/conversions-inl.h"
 #include "src/conversions.h"
@@ skipping 292 matching lines @@
 void PreParser::DeclareAndInitializeVariables(
     PreParserStatement block,
     const DeclarationDescriptor* declaration_descriptor,
     const DeclarationParsingResult::Declaration* declaration,
     ZoneList<const AstRawString*>* names, bool* ok) {
   if (declaration->pattern.variables_ != nullptr) {
     DCHECK(FLAG_lazy_inner_functions);
     DCHECK(track_unresolved_variables_);
     for (auto variable : *(declaration->pattern.variables_)) {
       declaration_descriptor->scope->RemoveUnresolved(variable);
-      scope()->DeclareVariableName(variable->raw_name(),
-                                   declaration_descriptor->mode);
+      Variable* var = scope()->DeclareVariableName(
+          variable->raw_name(), declaration_descriptor->mode);
+      if (FLAG_preparser_scope_analysis) {
+        MarkLoopVariableAsAssigned(declaration_descriptor->scope, var);
+        // This is only necessary if there is an initializer, but we don't have
+        // that information here.  Consequently, the preparser sometimes says
+        // maybe-assigned where the parser (correctly) says never-assigned.
+      }
       if (names) {
         names->Add(variable->raw_name(), zone());
       }
     }
   }
 }
 
 #undef CHECK_OK
 #undef CHECK_OK_CUSTOM
 
 
 }  // namespace internal
 }  // namespace v8