Add mergeWith (ReactiveX#5252)

* feat(mergeWith): add mergeWith

* test(mergeWith): update tests

spec-dtslint/operators/mergeWith-spec.ts

@@ -0,0 +1,13 @@
+import { mergeWith } from 'rxjs/operators';
+import { a$, b$, c$, d$, e$, f$, g$, h$} from '../helpers';
+
+it('should accept N args', () => {
+  const r0 = a$.pipe(mergeWith()); // $ExpectType Observable<A>
+  const r1 = a$.pipe(mergeWith(b$)); // $ExpectType Observable<A | B>
+  const r2 = a$.pipe(mergeWith(b$, c$)); // $ExpectType Observable<A | B | C>
+  const r3 = a$.pipe(mergeWith(b$, c$, d$)); // $ExpectType Observable<A | B | C | D>
+  const r4 = a$.pipe(mergeWith(b$, c$, d$, e$)); // $ExpectType Observable<A | B | C | D | E>
+  const r5 = a$.pipe(mergeWith(b$, c$, d$, e$, f$)); // $ExpectType Observable<A | B | C | D | E | F>
+  const r6 = a$.pipe(mergeWith(b$, c$, d$, e$, f$, g$)); // $ExpectType Observable<A | B | C | D | E | F | G>
+  const r7 = a$.pipe(mergeWith(b$, c$, d$, e$, f$, g$, h$)); // $ExpectType Observable<A | B | C | D | E | F | G | H>
+});

spec/operators/merge-legacy-spec.ts

@@ -0,0 +1,23 @@
+import { merge } from 'rxjs/operators';
+import { queueScheduler, of } from 'rxjs';
+import { expect } from 'chai';
+
+describe('merge (legacy)', () => {
+  it('should merge an immediately-scheduled source with an immediately-scheduled second', done => {
+    const a = of(1, 2, 3, queueScheduler);
+    const b = of(4, 5, 6, 7, 8, queueScheduler);
+    const r = [1, 2, 4, 3, 5, 6, 7, 8];
+
+    a.pipe(merge(b, queueScheduler)).subscribe(
+      val => {
+        expect(val).to.equal(r.shift());
+      },
+      x => {
+        done(new Error('should not be called'));
+      },
+      () => {
+        done();
+      }
+    );
+  });
+});