Better error for erroneous trackBy: expressions

[MichaelRFairhurst]

In playground/outline_component.html I put in the line

<li *ngFor='let item of items; var x=index; trackBy: items' [hidden]='item != null'>

and I get the very confusing error of

WARNING: Attribute value expression (of type List<String>) is not assignable to component input (of type (num, dynamic) → dynamic) ([angular2_quickstart] lib/overview_component.html:1)

Could use some dedicated logic to clean this error up