feat(ngFor): add custom trackBy function support

[kara]

Make it possible to track items in iterables in custom ways (e.g. by ID or index), rather than simply by identity.

Usage:

With template element:

@Component(
  template: `
   <template ngFor #item [ngForOf]="items" [ngForTrackBy]=customTrackBy">
      {{item}}
   </template>
   `
)
class MyComponent {
  customTrackBy(index: number, obj: any): any {
    return index;
  }
}

With *ngFor:

@Component(
  template: `
   <div *ngFor="#item of items; trackBy:customTrackBy">
      {{item}}
   </div>
   `
)
class MyComponent {
  customTrackBy(index: number, obj: any): any {
    return index;
  }
}

@mhevery Did you want to review this?

Closes #4402

[vsavkin]

Can you do _trackByFn: TrackByFn = trackByIdentity instead? Try to do this if ts2dart supports it. If not, what you have is good.

[kara]

Yeah, that's what I wanted to do too :) It doesn't seem to be supported.

[mary-poppins]

Merging PR #6779 on behalf of @alexeagle to branch presubmit-alexeagle-pr-6779.

[johnpapa]

linking to the new issue #6872

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