[js-api] Handle thenable Module in instantiate(bytes)

document/js-api/index.bs

@@ -641,6 +641,7 @@ The verification of WebAssembly type requirements is deferred to the
     1. Let |promise| be [=a new promise=].
     1. [=React=] to |promiseOfModule|:
         * If |promiseOfModule| was fulfilled with value |module|:
+            1. If |module| does not [=implement=] {{Module}}, [=Reject=] |promise| with a {{TypeError}} exception and terminate these substeps.
             1. [=asynchronously instantiate a WebAssembly module|Instantiate the WebAssembly module=] |module| importing |importObject|, and let |innerPromise| be the result.
             1. [=React=] to |innerPromise|:
                 * If |innerPromise| was fulfilled with value |instance|.

test/js-api/constructor/instantiate.any.js

@@ -150,3 +150,35 @@ promise_test(() => {
   buffer[0] = 1;
   return promise.then(assert_WebAssemblyInstantiatedSource);
 }, "Changing the buffer");
+
+promise_test(t => {
+  const originalThen = WebAssembly.Module.prototype.then;
+  const otherModule = new WebAssembly.Module(emptyModuleBinary);
+  t.add_cleanup(() => {
+    WebAssembly.Module.prototype.then = originalThen;
+  });
+  let thenCalledCount = 0;
+  WebAssembly.Module.prototype.then = function(resolve) {
+    thenCalledCount++;
+    WebAssembly.Module.prototype.then = originalThen;
+    resolve(otherModule);
+  };
+  return WebAssembly.instantiate(emptyModuleBinary).then(result => {
+    assert_equals(thenCalledCount, 1, "then was called exactly once");
+    assert_WebAssemblyInstantiatedSource(result);
+    assert_equals(result.module, otherModule, "The module from 'then' was used");
+  });
+}, "instantiate(bytes) with a thenable WebAssembly.Module returning a different Module");
+
+promise_test(t => {
+  const originalThen = WebAssembly.Module.prototype.then;
+  t.add_cleanup(() => {
+    if (originalThen === undefined) {
+      delete WebAssembly.Module.prototype.then;
+    } else {
+      WebAssembly.Module.prototype.then = originalThen;
+    }
+  });
+  WebAssembly.Module.prototype.then = (resolve) => resolve(17);
+  return promise_rejects_js(t, TypeError, WebAssembly.instantiate(emptyModuleBinary));
+}, "instantiate(bytes) with a thenable WebAssembly.Module returning a non-Module value");