test(mapTo): add test for mapTo expected behavior

tests/operator/mapTo.ts

@@ -0,0 +1,21 @@
+import xs from '../../src/index';
+import * as assert from 'assert';
+
+describe('Stream.prototype.mapTo', () => {
+  it('should transform events to a constant value', (done) => {
+    const stream = xs.interval(100).mapTo(10);
+    const expected = [10, 10, 10];
+    let listener = {
+      next: (x: number) => {
+        assert.equal(x, expected.shift());
+        if (expected.length === 0) {
+          stream.removeListener(listener);
+          done();
+        }
+      },
+      error: done.fail,
+      complete: done.fail,
+    };
+    stream.addListener(listener);
+  });
+});