DOM: Implement the last() Observable operator

See WICG/observable#133. This CL implements the
`last()` operator which returns a Promise that resolves to the last
item that the source Observable emits, and rejects with a RangeError
if the Observable completes prematurely.

For WPTs:
Co-authored-by: [email protected]

[email protected]

Bug: 40282760
Change-Id: Id1ec2af152c288ae81dfe383dfa9a7f166cfb375
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5417373
Reviewed-by: Di Zhang <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1282037}

dom/observable/tentative/observable-last.any.js

@@ -0,0 +1,113 @@
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    // Never exposed to the `last()` promise.
+    subscriber.next(1);
+
+    subscriber.next(2);
+    subscriber.complete();
+  });
+
+  const value = await source.last();
+
+  assert_equals(value, 2);
+}, "last(): Promise resolves to last value");
+
+promise_test(async () => {
+  const error = new Error("error from source");
+  const source = new Observable(subscriber => {
+    subscriber.error(error);
+  });
+
+  let rejection = null;
+  try {
+    await source.last();
+  } catch (e) {
+    rejection = e;
+  }
+
+  assert_equals(rejection, error);
+}, "last(): Promise rejects with emitted error");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.complete();
+  });
+
+  let rejection = null;
+  try {
+    await source.last();
+  } catch (e) {
+    rejection = e;
+  }
+
+  assert_true(rejection instanceof RangeError,
+      "Promise rejects with RangeError");
+  assert_equals(rejection.message, "No values in Observable");
+}, "last(): Promise rejects with RangeError when source Observable " +
+   "completes without emitting any values");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {});
+
+  const controller = new AbortController();
+  const promise = source.last({ signal: controller.signal });
+
+  controller.abort();
+
+  let rejection = null;
+  try {
+    await promise;
+  } catch (e) {
+    rejection = e;
+  }
+
+  assert_true(rejection instanceof DOMException,
+      "Promise rejects with a DOMException for abortion");
+  assert_equals(rejection.name, "AbortError",
+      "Rejected with 'AbortError' DOMException");
+  assert_equals(rejection.message, "signal is aborted without reason");
+}, "last(): Aborting a signal rejects the Promise with an AbortError DOMException");
+
+promise_test(async () => {
+  const results = [];
+  const source = new Observable(subscriber => {
+    results.push("source subscribe");
+    subscriber.addTeardown(() => results.push("source teardown"));
+    subscriber.signal.addEventListener("abort", () => results.push("source abort"));
+    results.push("before source next 1");
+    subscriber.next(1);
+    results.push("after source next 1");
+    results.push("before source complete");
+    subscriber.complete();
+    results.push("after source complete");
+  });
+
+  results.push("calling last");
+  const promise = source.last();
+
+  assert_array_equals(results, [
+    "calling last",
+    "source subscribe",
+    "before source next 1",
+    "after source next 1",
+    "before source complete",
+    "source teardown",
+    "source abort",
+    "after source complete",
+  ], "Array values after last() is called");
+
+  const lastValue = await promise;
+  results.push(`last resolved with: ${lastValue}`);
+
+  assert_array_equals(results, [
+    "calling last",
+    "source subscribe",
+    "before source next 1",
+    "after source next 1",
+    "before source complete",
+    "source teardown",
+    "source abort",
+    "after source complete",
+    "last resolved with: 1",
+  ], "Array values after Promise is awaited");
+}, "last(): Lifecycle");