[proxy] fixing harmony/proxy.js tests and improving error messages + some drive-by fixes

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 961cefa4f52404982174e120bc40ede50f7e1538..ff0576a96f7398f351418ed587fc1b1c88771195 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -1096,11 +1096,6 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ cmpp(rdx, FieldOperand(rcx, HeapObject::kMapOffset));
   __ j(equal, &update_each, Label::kNear);
 
-  // For proxies, no filtering is done.
-  // TODO(rossberg): What if only a prototype is a proxy? Not specified yet.
-  __ Cmp(rdx, Smi::FromInt(0));
-  __ j(equal, &update_each, Label::kNear);
-
   // Convert the entry to a string or null if it isn't a property
   // anymore. If the property has been removed while iterating, we
   // just skip it.