fix: somewhat tighter test for plain empty object (#1981)

packages/ERTP/src/displayInfo.js

@@ -1,5 +1,10 @@
 import { assert, details, q } from '@agoric/assert';
-import { pureCopy, passStyleOf, REMOTE_STYLE } from '@agoric/marshal';
+import {
+  pureCopy,
+  passStyleOf,
+  REMOTE_STYLE,
+  getInterfaceOf,
+} from '@agoric/marshal';
 
 // TODO: assertSubset and assertKeysAllowed are copied from Zoe. Move
 // this code to a location where it can be used by ERTP and Zoe
@@ -44,6 +49,18 @@ export const assertDisplayInfo = allegedDisplayInfo => {
 
 export const coerceDisplayInfo = allegedDisplayInfo => {
   if (passStyleOf(allegedDisplayInfo) === REMOTE_STYLE) {
+    // These condition together try to ensure that `allegedDisplayInfo`
+    // is a plain empty object. It will accept all plain empty objects
+    // that it should. It will reject most things we want to reject including
+    // remotables that are explicitly declared `Remotable`. But a normal
+    // HandledPromise presence not explicitly declared `Remotable` will
+    // be mistaken for a plain empty object. Even in this case, the copy
+    // has a new identity, so the only danger is that we didn't reject
+    // with a diagnostic, potentially masking a programmer error.
+    assert(Object.isFrozen(allegedDisplayInfo));
+    assert.equal(Reflect.ownKeys(allegedDisplayInfo).length, 0);
+    assert.equal(Object.getPrototypeOf(allegedDisplayInfo), Object.prototype);
+    assert.equal(getInterfaceOf(allegedDisplayInfo), undefined);
     return harden({});
   }
   allegedDisplayInfo = pureCopy(allegedDisplayInfo);