feat(zoe): error on unexpected properties from start()

packages/zoe/src/contractFacet/zcfZygote.js

@@ -379,7 +379,14 @@ export const makeZCFZygote = async (
           creatorFacet = undefined,
           publicFacet = undefined,
           creatorInvitation = undefined,
+          ...unexpected
         }) => {
+          const unexpectedKeys = Object.keys(unexpected);
+          unexpectedKeys.length === 0 ||
+            Fail`contract ${
+              prepare ? 'prepare' : 'start'
+            } returned unrecognized properties ${unexpectedKeys}`;
+
           const areDurable = objectMap(
             { creatorFacet, publicFacet, creatorInvitation },
             canBeDurable,

packages/zoe/test/unitTests/test-zoe.js

@@ -172,6 +172,19 @@ test(`E(zoe).startInstance - bad issuer, makeEmptyPurse throws`, async t => {
   );
 });
 
+test(`E(zoe).startInstance - unexpected properties`, async t => {
+  const { zoe } = setup();
+
+  const contractPath = `${dirname}/unexpectedPropertiesContract.js`;
+  const bundle = await bundleSource(contractPath);
+  const installation = await E(zoe).install(bundle);
+
+  await t.throwsAsync(() => E(zoe).startInstance(installation), {
+    message:
+      'contract "start" returned unrecognized properties ["unexpectedProperty"]',
+  });
+});
+
 test(`E(zoe).offer`, async t => {
   const { zoe, zcf } = await setupZCFTest();
   const invitation = zcf.makeInvitation(() => 'result', 'invitation');

packages/zoe/test/unitTests/unexpectedPropertiesContract.js

@@ -0,0 +1 @@
+export const start = () => ({ unexpectedProperty: {} });