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

src/full-codegen/mips/full-codegen-mips.cc

Index: src/full-codegen/mips/full-codegen-mips.cc
diff --git a/src/full-codegen/mips/full-codegen-mips.cc b/src/full-codegen/mips/full-codegen-mips.cc
index 14ff5c3443d893ab4c09bb8a8bbae60505d25231..a3d3e61d4a9285e74b76eb05bb13a3e9fe4dc86b 100644
--- a/src/full-codegen/mips/full-codegen-mips.cc
+++ b/src/full-codegen/mips/full-codegen-mips.cc
@@ -1138,11 +1138,6 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ lw(t0, FieldMemOperand(a1, HeapObject::kMapOffset));
   __ Branch(&update_each, eq, t0, Operand(a2));
 
-  // For proxies, no filtering is done.
-  // TODO(rossberg): What if only a prototype is a proxy? Not specified yet.
-  DCHECK_EQ(static_cast<Smi*>(0), Smi::FromInt(0));
-  __ Branch(&update_each, eq, a2, Operand(zero_reg));
-
   // Convert the entry to a string or (smi) 0 if it isn't a property
   // any more. If the property has been removed while iterating, we
   // just skip it.