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

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index 2f5c52098dca0adcdffb9f5e730b46de9e492f05..02dce6bdf632433bae4717ffb2e8add156867d0e 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -1799,9 +1799,18 @@ BUILTIN(ProxyConstructor) {
 BUILTIN(ProxyConstructor_ConstructStub) {
   HandleScope scope(isolate);
   DCHECK(isolate->proxy_function()->IsConstructor());
-  DCHECK_EQ(3, args.length());
-  Handle<Object> target = args.at<Object>(1);
-  Handle<Object> handler = args.at<Object>(2);
+  Handle<Object> target;
+  if (args.length() < 2) {
+    target = isolate->factory()->undefined_value();
+  } else {
+    target = args.at<Object>(1);
+  }
+  Handle<Object> handler;
+  if (args.length() < 3) {
+    handler = isolate->factory()->undefined_value();
+  } else {
+    handler = args.at<Object>(2);
+  }
   // The ConstructStub is executed in the context of the caller, so we need
   // to enter the callee context first before raising an exception.
   isolate->set_context(args.target()->context());