DOM: Implement the filter() Observable operator

This CL implements the semantics specified in
https://wicg.github.io/observable/#dom-observable-filter.

See WICG/observable#112.

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

[email protected]

Bug: 40282760
Change-Id: I507cd7bd4117e81e718bb31e496cc4b763d9570a

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

@@ -0,0 +1,105 @@
+test(() => {
+  const source = new Observable(subscriber => {
+    subscriber.next(1);
+    subscriber.next(2);
+    subscriber.next(3);
+    subscriber.next(4);
+    subscriber.complete();
+  });
+
+  const results = [];
+
+  source
+    .filter(value => value % 2 === 0)
+    .subscribe({
+      next: v => results.push(v),
+      error: () => results.push("error"),
+      complete: () => results.push("complete"),
+    });
+
+  assert_array_equals(results, [2, 4, "complete"]);
+}, "filter(): Returned Observable filters out results based on predicate");
+
+test(() => {
+  const error = new Error("error while filtering");
+  const results = [];
+  let teardownCalled = false;
+
+  const source = new Observable(subscriber => {
+    subscriber.addTeardown(() => teardownCalled = true);
+    subscriber.next(1);
+    assert_true(teardownCalled, "Teardown called once map unsubscribes due to error");
+    assert_false(subscriber.active, "Unsubscription makes Subscriber inactive");
+    subscriber.next(2);
+    subscriber.complete();
+  });
+
+  source
+    .filter(() => {
+      throw error;
+    })
+    .subscribe({
+      next: v => results.push(v),
+      error: e => results.push(e),
+      complete: () => results.push("complete"),
+    });
+
+  assert_array_equals(results, [error]);
+}, "filter(): Errors thrown in filter predicate are emitted to Observer error() handler");
+
+test(() => {
+  const source = new Observable(subscriber => {
+    subscriber.next(1);
+    subscriber.complete();
+    subscriber.next(2);
+  });
+
+  let predicateCalls = 0;
+  const results = [];
+  source.filter(v => ++predicateCalls).subscribe({
+    next: v => results.push(v),
+    error: e => results.push(e),
+    complete: () => results.push('complete'),
+  });
+
+  assert_equals(predicateCalls, 1, "Predicate is not called after complete()");
+  assert_array_equals(results, [1, "complete"]);
+}, "filter(): Passes complete() through from source Observable");
+
+test(() => {
+  const source = new Observable(subscriber => {
+    subscriber.next(1);
+    subscriber.error('error');
+    subscriber.next(2);
+  });
+
+  let predicateCalls = 0;
+  const results = [];
+  source.map(v => ++predicateCalls).subscribe({
+    next: v => results.push(v),
+    error: e => results.push(e),
+    complete: () => results.push('complete'),
+  });
+
+  assert_equals(predicateCalls, 1, "Predicate is not called after error()");
+  assert_array_equals(results, [1, "error"]);
+}, "filter(): Passes error() through from source Observable");
+
+test(() => {
+  const results = [];
+  const source = new Observable(subscriber => {
+    subscriber.addTeardown(() => results.push('source teardown'));
+    subscriber.signal.addEventListener('abort',
+        () => results.push('source abort event'));
+
+    subscriber.complete();
+  });
+
+  source.filter(() => results.push('filter predicate called')).subscribe({
+    complete: () => results.push('filter observable complete'),
+  });
+
+  assert_array_equals(results,
+      ['source teardown', 'source abort event', 'filter observable complete']);
+}, "filter(): Upon source completion, source Observable teardown sequence " +
+   "happens after downstream filter complete() is called");