perf(core): refactor NgZone, decrease size by 1.2Kb (#17773)

- Remove getters
- Hide private methods for better property renaming

```
497893 May 31 11:26 core.umd.js
718073 May 31 11:26 core.umd.js.map
217108 May 31 11:26 core.umd.min.js
575092 May 31 11:26 core.umd.min.js.map
```

```
495594 May 31 11:28 core.umd.js
716943 May 31 11:28 core.umd.js.map
215826 May 31 11:28 core.umd.min.js
574401 May 31 11:28 core.umd.min.js.map
```
diff: 1,282

packages/core/testing/src/ng_zone_mock.ts

@@ -14,12 +14,10 @@ import {EventEmitter, Injectable, NgZone} from '@angular/core';
  */
 @Injectable()
 export class MockNgZone extends NgZone {
-  private _mockOnStable: EventEmitter<any> = new EventEmitter(false);
+  onStable: EventEmitter<any> = new EventEmitter(false);
 
   constructor() { super({enableLongStackTrace: false}); }
 
-  get onStable() { return this._mockOnStable; }
-
   run(fn: Function): any { return fn(); }
 
   runOutsideAngular(fn: Function): any { return fn(); }