[Navigator] Add a callback that is called after emitting an event.

Summary:
While adeveloper requests the emitter to emit an event, the emitter
may not emit the event immediately instead of putting the request
into a queue and process it later.

This diff allows the developer to provide a callback which will be called
when the event has been emitted.

For instance:

```
class NavigationContext {
  push(nextRoute) {
    var nextStack = this._stack.push(nextRoute);
    this.emit(
      'change',
      {
        reason: 'push',
        nextStack: nextStack,
        nextRoute: nextRoute,
      },
      this._onPush
    );
  }

  _onPush(event){
    if (event.defaultPrevented) {
      return;
    }
    this._stack = event.nextStack;
    this.emit('change');
  }
}
```

Libraries/CustomComponents/Navigator/Navigation/__tests__/NavigationEventEmitter-test.js

@@ -34,27 +34,48 @@ jest
 var NavigationEventEmitter = require('NavigationEventEmitter');
 
 describe('NavigationEventEmitter', () => {
-  it('emit event', () => {
-    var target = {};
-    var emitter = new NavigationEventEmitter(target);
-    var focusCounter = 0;
-    var focusTarget;
+  it('emits event', () => {
+    var context = {};
+    var emitter = new NavigationEventEmitter(context);
+    var logs = [];
+
+    emitter.addListener('ping', (event) => {
+      var {type, data, target, defaultPrevented} = event;
+
+      logs.push({
+        data,
+        defaultPrevented,
+        target,
+        type,
+      });
 
-    emitter.addListener('focus', (event) => {
-      focusCounter++;
-      focusTarget = event.target;
     });
 
-    emitter.emit('focus');
-    emitter.emit('blur');
+    emitter.emit('ping', 'hello');
 
-    expect(focusCounter).toBe(1);
-    expect(focusTarget).toBe(target);
+    expect(logs.length).toBe(1);
+    expect(logs[0].target).toBe(context);
+    expect(logs[0].type).toBe('ping');
+    expect(logs[0].data).toBe('hello');
+    expect(logs[0].defaultPrevented).toBe(false);
   });
 
-  it('put nested emit call in queue', () => {
-    var target = {};
-    var emitter = new NavigationEventEmitter(target);
+  it('does not emit event that has no listeners', () => {
+    var context = {};
+    var emitter = new NavigationEventEmitter(context);
+    var pinged = false;
+
+    emitter.addListener('ping', () => {
+      pinged = true;
+    });
+
+    emitter.emit('yo', 'bo');
+    expect(pinged).toBe(false);
+  });
+
+  it('puts nested emit call in a queue', () => {
+    var context = {};
+    var emitter = new NavigationEventEmitter(context);
     var logs = [];
 
     emitter.addListener('one', () => {
@@ -77,4 +98,63 @@ describe('NavigationEventEmitter', () => {
 
     expect(logs).toEqual([1, 2, 3, 4, 5]);
   });
+
+  it('calls callback after emitting', () => {
+    var context = {};
+    var emitter = new NavigationEventEmitter(context);
+    var logs = [];
+
+    emitter.addListener('ping', (event) => {
+      var {type, data, target, defaultPrevented} = event;
+      logs.push({
+        data,
+        defaultPrevented,
+        target,
+        type,
+      });
+      event.preventDefault();
+    });
+
+    emitter.emit('ping', 'hello', (event) => {
+      var {type, data, target, defaultPrevented} = event;
+      logs.push({
+        data,
+        defaultPrevented,
+        target,
+        type,
+      });
+    });
+
+    expect(logs.length).toBe(2);
+    expect(logs[1].target).toBe(context);
+    expect(logs[1].type).toBe('ping');
+    expect(logs[1].data).toBe('hello');
+    expect(logs[1].defaultPrevented).toBe(true);
+  });
+
+  it('calls callback after emitting the current event and before ' +
+       'emitting the next event', () => {
+    var context = {};
+    var emitter = new NavigationEventEmitter(context);
+    var logs = [];
+
+    emitter.addListener('ping', (event) => {
+      logs.push('ping');
+      emitter.emit('pong');
+    });
+
+    emitter.addListener('pong', (event) => {
+      logs.push('pong');
+    });
+
+    emitter.emit('ping', null, () => {
+      logs.push('did-ping');
+    });
+
+    expect(logs).toEqual([
+      'ping',
+      'did-ping',
+      'pong',
+    ]);
+  });
 });