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

src/full-codegen/mips64/full-codegen-mips64.cc

Index: src/full-codegen/mips64/full-codegen-mips64.cc
diff --git a/src/full-codegen/mips64/full-codegen-mips64.cc b/src/full-codegen/mips64/full-codegen-mips64.cc
index 4979132ec6610816e802ec135bb0c9c04f2512a6..6aa5f989033238b15de393d6601831ece64a2913 100644
--- a/src/full-codegen/mips64/full-codegen-mips64.cc
+++ b/src/full-codegen/mips64/full-codegen-mips64.cc
@@ -1138,11 +1138,6 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ ld(a4, FieldMemOperand(a1, HeapObject::kMapOffset));
   __ Branch(&update_each, eq, a4, 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.