Close angular#40215

`fesm2015/zone.js` is built to `esm` bundle with rollup, so the 'use strict';
statement is not generated in the bundle, even we put the 'use strict' in the src code,
rollup removes the code in the final bundle.

So if we load the `fesm2015/zone.js` as a module, such as `ng serve`, in the index.html

```
<script src="polyfills.js" type="module"></script>
```

Everything works fine, since polyfills.js is loaded as `module`, so it is always `strict`.

But in `ng test`, webpack concat the `zone.js` and loaded into the karma html. For other app and
 test code, they are still `strict` since they are `module` because they have `export/import`
statement, but `zone.js` is a bundle without `export`, it is a `side effect` bundle, so after
 loaded by webpack, it becomes non-strict. Which causes some issues, such as angular#40215,
the root cause is the `this` context should be `undefined` but treated as `Window` in `non-strict` mode.

```
Object.prototype.toString.apply(undefined);
// should be [object undefined], but it is [object Window] in non-strict mode.

// zone.js patched version of toString
Object.prototype.toString = function() {
  ...
  // in non-strict mode, this is Window
  return originalObjectPrototypeToString.call(this);
}

```

So in this commit, `'use strict';` is always added to the `esm` bundles.

…ndles