Add Then Finally Functions test

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

@@ -0,0 +1,30 @@
+// Copyright (C) 2019 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-thenfinallyfunctions
+description: >
+  valueThunk is anonymous built-in function with length of 1 that returns value.
+info: |
+  Then Finally Functions
+
+  ...
+  8. Let valueThunk be equivalent to a function that returns value.
+  9. Return ? Invoke(promise, "then", « valueThunk »).
+
+  The "length" property of a Then Finally function is 1.
+features: [Promise.prototype.finally, Reflect.construct]
+includes: [isConstructor.js]
+flags: [async]
+---*/
+
+var value = {};
+
+Promise.resolve(value).finally(function() {});
+Promise.prototype.then = function(valueThunk) {
+  assert(!isConstructor(valueThunk));
+  assert.sameValue(valueThunk.length, 1);
+  assert.sameValue(valueThunk.name, '');
+  assert.sameValue(valueThunk(), value);
+
+  $DONE();
+};