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

src/parsing/preparser.cc

Index: src/parsing/preparser.cc
diff --git a/src/parsing/preparser.cc b/src/parsing/preparser.cc
index 0377a1b9d35e70ff9cb131ad061e01f7f257888f..43877b030c764c09332f6fad89e3f1b4d806c609 100644
--- a/src/parsing/preparser.cc
+++ b/src/parsing/preparser.cc
@@ -310,8 +310,14 @@
     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());
       }