Promise.prototype.finally calls PromiseResolved: add resolved promise test

test/built-ins/Promise/prototype/finally/resolved-observable-then-calls-PromiseResolve.js

@@ -0,0 +1,45 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-thenfinallyfunctions
+description: >
+  PromiseResolve() avoids extra Promise capability creation.
+info: |
+  Then Finally Functions
+
+  [...]
+  7. Let promise be ? PromiseResolve(C, result).
+  8. Let valueThunk be equivalent to a function that returns value.
+  9. Return ? Invoke(promise, "then", « valueThunk »).
+
+  PromiseResolve ( C, x )
+
+  1. Assert: Type(C) is Object.
+  2. If IsPromise(x) is true, then
+    a. Let xConstructor be ? Get(x, "constructor").
+    b. If SameValue(xConstructor, C) is true, return x.
+features: [Promise.prototype.finally]
+flags: [async]
+---*/
+
+class MyPromise extends Promise {}
+
+var mp1Value = {};
+var mp1 = MyPromise.resolve(mp1Value);
+var mp2 = MyPromise.resolve(42);
+
+var thenCalls = [];
+var then = Promise.prototype.then;
+Promise.prototype.then = function(resolve, reject) {
+  thenCalls.push({promise: this, resolve, reject});
+  return then.call(this, resolve, reject);
+};
+
+mp1.finally(() => mp2).then(() => {
+  assert.sameValue(thenCalls.length, 5);
+
+  var mp2Calls = thenCalls.filter(c => c.promise === mp2);
+  assert.sameValue(mp2Calls.length, 1);
+  assert.sameValue(mp2Calls[0].reject, undefined);
+  assert.sameValue(mp2Calls[0].resolve(), mp1Value);
+}).then($DONE, $DONE);