fix(zone.js): correctly patch es6 classes

[edusperoni]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included.

What is the current behavior?

Zone uses a for(prop in object) to patch all class properties, but this doesn't work with es6 as class properties aren't enumerable. Since NativeScript is distributed in es6+ the FileReader polyfill doesn't get properly patched by zonejs

What is the new behavior?

Use Object.getOwnPropertyNames instead, so FileReader is patched correctly.

Currently only MutationObserver, WebKitMutationObserver, IntersectionObserver, FileReader and legacy XMLHttpRequest are patched this way. Of all of them, only FileReader is polyfilled by NativeScript (the legacy XMLHttpRequest patch is already disabled)

Fixes #14.