[proxies] Improve JSProxyVerify and test most proxy invariants.

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index e7ee7eca7731c3fe9593055f4fc2906b7ec1dfd5..6d99a6f2277e1e0edd31174264b986a3cd04becc 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -818,8 +818,14 @@ void JSRegExp::JSRegExpVerify() {
 
 void JSProxy::JSProxyVerify() {
   CHECK(IsJSProxy());
+  VerifyPointer(target());
   VerifyPointer(handler());
+  CHECK_EQ(target()->IsCallable(), map()->is_callable());
+  CHECK_EQ(target()->IsConstructor(), map()->is_constructor());
   CHECK(hash()->IsSmi() || hash()->IsUndefined());
+  CHECK(map()->prototype()->IsNull());
+  // There should be no properties on a Proxy.
+  CHECK_EQ(0, map()->NumberOfOwnDescriptors());
 }