chore(ts): fix TODOs that were pending on 1.6 upgrade

Closes #4377

modules/angular2/src/core/forms/directives/ng_control.ts

@@ -2,10 +2,13 @@ import {ControlValueAccessor} from './control_value_accessor';
 import {AbstractControlDirective} from './abstract_control_directive';
 
 /**
- * An abstract class that all control directive extend.
- *
+ * A base class that all control directive extend.
  * It binds a {@link Control} object to a DOM element.
  */
+// Cannot currently be abstract because it would contain
+// an abstract method in the public API, and we cannot reflect
+// on that in Dart due to https://github.com/dart-lang/sdk/issues/18721
+// Also we don't have abstract setters, see https://github.com/Microsoft/TypeScript/issues/4669
 export class NgControl extends AbstractControlDirective {
   name: string = null;
   valueAccessor: ControlValueAccessor = null;