[proxies] Add missing condition to GetProperty consistency check.

BUG=v8:1543
LOG=N
R=cbruni@chromium.org,jkummerow@chromium.org

Review URL: https://codereview.chromium.org/1519073002

Cr-Commit-Position: refs/heads/master@{#32832}

src/objects.cc

@@ -805,7 +805,8 @@ MaybeHandle<Object> JSProxy::GetProperty(Isolate* isolate,
     inconsistent =
         inconsistent ||
         (PropertyDescriptor::IsAccessorDescriptor(&target_desc) &&
-         !target_desc.configurable() && target_desc.get()->IsUndefined());
+         !target_desc.configurable() && target_desc.get()->IsUndefined() &&
+         !trap_result->IsUndefined());
     if (inconsistent) {
       THROW_NEW_ERROR(
           isolate,

test/mjsunit/harmony/proxies-get.js

@@ -52,7 +52,7 @@
 
 (function testFailingInvariant() {
   var target = {};
-  var handler = { get: function(){ return "value" }}
+  var handler = { get: function(r, p){ if (p != "key4") return "value" }}
   var proxy = new Proxy(target, handler);
   assertEquals("value", proxy.property);
   assertEquals("value", proxy.key);
@@ -92,6 +92,14 @@
   assertThrows(function(){ proxy.key }, TypeError);
   assertEquals("value", proxy.key2);
   assertThrows(function(){ proxy.key3 }, TypeError);
+
+  // Define a non-configurable setter without a corresponding getter on the
+  // target for which the handler will return undefined.
+  Object.defineProperty(target, "key4", {
+    configurable: false,
+    set: function() { }
+  });
+  assertSame(undefined, proxy.key4);
 })();
 
 (function testGetInternalIterators() {