[es6] Handle conflicts for sloppy let

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index aa866647d1eb99ad426b5f7e9dbd116a531edd6f..2f1e740d1935d02c1a3d7740ec9c4b76e6c185e2 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -3765,6 +3765,8 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
     if (is_strict(language_mode())) {
       CheckStrictOctalLiteral(formal_parameters.scope->start_position(),
                               scanner()->location().end_pos, CHECK_OK);
+    }
+    if (is_strict(language_mode()) || allow_harmony_sloppy()) {

[Dan Ehrenberg, 2015/07/08 23:42:13]

A tangent, but it looks like CheckConflictingVarDeclarations is implemented only
in the parser, not the preparser, so the errors aren't quite as early as we'd
optimally like. Any particular reason for this? It looks like the preparser
maintains the whole scope thing anyway.

[rossberg, 2015/07/10 12:13:17]

No, the preparser doesn't maintain scopes. This is a long-standing issue. We
would love to do that, but experiments showed that it is fairly costly and
causes significant regressions in e.g. CodeLoad. It's still an open problem how
to diagnose binding errors early in all cases, and we have punted so far.

(It's a particular pain point for strong mode, btw.)

[arv (Not doing code reviews), 2015/07/10 13:00:48]

This is not entirely true. We do set up the scopes but we do not create the
bindings in them in the preparser.

       this->CheckConflictingVarDeclarations(formal_parameters.scope, CHECK_OK);
     }
   }