Promise.prototype.finally: add missing tests

Per:
 - https://github.com/tc39/test262/issues/866#issuecomment-328565337
 - https://github.com/tc39/test262/issues/866#issuecomment-359978873

Closes #866.

test/built-ins/Promise/prototype/finally/species-constructor.js

@@ -2,7 +2,7 @@
 // This code is governed by the BSD license found in the LICENSE file.
 /*---
 author: Sathya Gunasekaran
-description: finally calls the SpeciesConstructor
+description: finally calls the SpeciesConstructor and creates the right amount of promises
 esid: sec-promise.prototype.finally
 features: [Promise.prototype.finally]
 flags: [async]
@@ -22,4 +22,4 @@ new FooPromise(r => r())
   .then(() => {
     assert.sameValue(count, 6, "6 new promises were created");
     $DONE();
-});
+  }, $ERROR);

test/built-ins/Promise/prototype/finally/subclass-reject-count.js

+// Copyright (C) 2018 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+author: Jordan Harband
+description: Promise subclass finally on rejected creates the proper number of subclassed promises
+esid: sec-promise.prototype.finally
+features: [Promise.prototype.finally]
+flags: [async]
+---*/
+
+var count = 0;
+class FooPromise extends Promise {
+  constructor(resolve, reject) {
+    count++;
+    return super(resolve, reject);
+  }
+}
+
+FooPromise.reject().finally(() => {}).then($ERROR).catch(() => {
+  assert.sameValue(7, count);
+  $DONE();
+});

test/built-ins/Promise/prototype/finally/subclass-resolve-count.js

+// Copyright (C) 2018 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+author: Jordan Harband
+description: Promise subclass finally on resolved creates the proper number of subclassed promises
+esid: sec-promise.prototype.finally
+features: [Promise.prototype.finally]
+flags: [async]
+---*/
+
+var count = 0;
+class FooPromise extends Promise {
+  constructor(resolve, reject) {
+    count++;
+    return super(resolve, reject);
+  }
+}
+
+FooPromise.resolve().finally(() => {}).then(() => {
+  assert.sameValue(6, count);
+  $DONE();
+}, $ERROR);

test/built-ins/Promise/prototype/finally/subclass-species-constructor-reject-count.js

+// Copyright (C) 2018 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+author: Jordan Harband
+description: finally on rejected Promise calls the SpeciesConstructor
+esid: sec-promise.prototype.finally
+features: [Promise.prototype.finally]
+---*/
+
+class FooPromise extends Promise {
+  static get [Symbol.species]() { return Promise; }
+}
+
+var p = Promise.reject().finally(() => FooPromise.reject());
+
+assert.sameValue(p instanceof Promise, true);
+assert.sameValue(p instanceof FooPromise, false);

test/built-ins/Promise/prototype/finally/subclass-species-constructor-resolve-count.js

+// Copyright (C) 2018 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+author: Jordan Harband
+description: finally on resolved Promise calls the SpeciesConstructor
+esid: sec-promise.prototype.finally
+features: [Promise.prototype.finally]
+flags: [async]
+---*/
+
+class FooPromise extends Promise {
+  static get [Symbol.species]() { return Promise; }
+}
+
+var p = Promise.resolve().finally(() => FooPromise.resolve());
+
+assert.sameValue(p instanceof Promise, true);
+assert.sameValue(p instanceof FooPromise, false);

test/built-ins/Promise/prototype/finally/this-value-non-promise.js

+// Copyright (C) 2018 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+author: Jordan Harband
+description: >
+  Promise.prototype.finally called with a non-branded Promise does not throw
+esid: sec-promise.prototype.finally
+features: [Promise.prototype.finally]
+---*/
+
+var called = false;
+var p = new Proxy(Promise.resolve(), {});
+var oldThen = Promise.prototype.then;
+Promise.prototype.then = () => { called = true; };
+Promise.prototype.finally.call(p);
+assert.sameValue(called, true);
+Promise.prototype.then = oldThen;