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

src/full-codegen/x87/full-codegen-x87.cc

Index: src/full-codegen/x87/full-codegen-x87.cc
diff --git a/src/full-codegen/x87/full-codegen-x87.cc b/src/full-codegen/x87/full-codegen-x87.cc
index 2ac7a43cd9f1396a351ad96adba62b91c0e561dd..24c83b27f1e4c93c69c959bc425e8610b04009c3 100644
--- a/src/full-codegen/x87/full-codegen-x87.cc
+++ b/src/full-codegen/x87/full-codegen-x87.cc
@@ -1063,12 +1063,6 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ cmp(edx, FieldOperand(ecx, 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.
-  DCHECK(Smi::FromInt(0) == 0);
-  __ test(edx, edx);
-  __ j(zero, &update_each);
-
   // 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.