chore(typings): mark underscore methods @internal.

This allows TypeScript to produce an API surface which matches the Dart semantics.
I found these with:
gulp build.js.dev && find dist/js/dev/es5/angular2/src -name "*.d.ts" -exec grep -H -n '^ *_' {} \;

Closes #4638

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

@@ -58,6 +58,7 @@ const controlGroupBinding =
 })
 export class NgControlGroup extends ControlContainer implements OnInit,
     OnDestroy {
+  /** @internal */
   _parent: ControlContainer;
   constructor(@Host() @SkipSelf() _parent: ControlContainer) {
     super();

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

@@ -80,11 +80,13 @@ const controlNameBinding =
 })
 export class NgControlName extends NgControl implements OnChanges,
     OnDestroy {
+  /** @internal */
   _parent: ControlContainer;
   update = new EventEmitter();
   model: any;
   viewModel: any;
   validators: Function[];
+  /** @internal */
   _added = false;
 
   constructor(@Host() @SkipSelf() parent: ControlContainer,

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

@@ -157,10 +157,12 @@ export class NgForm extends ControlContainer implements Form {
     return false;
   }
 
+  /** @internal */
   _findContainer(path: string[]): ControlGroup {
     path.pop();
     return ListWrapper.isEmpty(path) ? this.form : <ControlGroup>this.form.find(path);
   }
 
+  /** @internal */
   _later(fn): void { PromiseWrapper.then(PromiseWrapper.resolve(null), fn, (_) => {}); }
 }

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

@@ -72,6 +72,7 @@ const formControlBinding =
 export class NgFormControl extends NgControl implements OnChanges {
   form: Control;
   update = new EventEmitter();
+  /** @internal */
   _added = false;
   model: any;
   viewModel: any;

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

@@ -140,6 +140,7 @@ export class NgFormModel extends ControlContainer implements Form,
     return false;
   }
 
+  /** @internal */
   _updateDomValue() {
     this.directives.forEach(dir => {
       var ctrl: any = this.form.find(dir.path);

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

@@ -41,7 +41,9 @@ const formControlBinding = CONST_EXPR(new Binding(NgControl, {toAlias: forwardRe
   exportAs: 'form'
 })
 export class NgModel extends NgControl implements OnChanges {
+  /** @internal */
   _control = new Control();
+  /** @internal */
   _added = false;
   update = new EventEmitter();
   model: any;

modules/angular2/src/core/forms/form_builder.ts

@@ -96,6 +96,7 @@ export class FormBuilder {
     }
   }
 
+  /** @internal */
   _reduceControls(controlsConfig: any): {[key: string]: modelModule.AbstractControl} {
     var controls: {[key: string]: modelModule.AbstractControl} = {};
     StringMapWrapper.forEach(controlsConfig, (controlConfig, controlName) => {
@@ -104,6 +105,7 @@ export class FormBuilder {
     return controls;
   }
 
+  /** @internal */
   _createControl(controlConfig: any): modelModule.AbstractControl {
     if (controlConfig instanceof modelModule.Control ||
         controlConfig instanceof modelModule.ControlGroup ||

modules/angular2/src/core/forms/model.ts

@@ -41,13 +41,19 @@ function _find(control: AbstractControl, path: Array<string | number>| string) {
  * Omitting from external API doc as this is really an abstract internal concept.
  */
 export class AbstractControl {
+  /** @internal */
   _value: any;
+  /** @internal */
   _status: string;
+  /** @internal */
   _errors: {[key: string]: any};
+  /** @internal */
   _pristine: boolean = true;
+  /** @internal */
   _touched: boolean = false;
+  /** @internal */
   _parent: ControlGroup | ControlArray;
-
+  /** @internal */
   _valueChanges: EventEmitter;
 
   constructor(public validator: Function) {}
@@ -128,6 +134,7 @@ export class AbstractControl {
     return isPresent(this.getError(errorCode, path));
   }
 
+  /** @internal */
   _updateValue(): void {}
 }
 
@@ -148,6 +155,7 @@ export class AbstractControl {
  * ### Example ([live demo](http://plnkr.co/edit/23DESOpbNnBpBHZt1BR4?p=preview))
  */
 export class Control extends AbstractControl {
+  /** @internal */
   _onChange: Function;
 
   constructor(value: any = null, validator: Function = Validators.nullValidator) {
@@ -234,12 +242,15 @@ export class ControlGroup extends AbstractControl {
     return c && this._included(controlName);
   }
 
+  /** @internal */
   _setParentForControls() {
     StringMapWrapper.forEach(this.controls, (control, name) => { control.setParent(this); });
   }
 
+  /** @internal */
   _updateValue() { this._value = this._reduceValue(); }
 
+  /** @internal */
   _reduceValue() {
     return this._reduceChildren({}, (acc, control, name) => {
       acc[name] = control.value;
@@ -247,6 +258,7 @@ export class ControlGroup extends AbstractControl {
     });
   }
 
+  /** @internal */
   _reduceChildren(initValue: any, fn: Function) {
     var res = initValue;
     StringMapWrapper.forEach(this.controls, (control, name) => {
@@ -257,6 +269,7 @@ export class ControlGroup extends AbstractControl {
     return res;
   }
 
+  /** @internal */
   _included(controlName: string): boolean {
     var isOptional = StringMapWrapper.contains(this._optionals, controlName);
     return !isOptional || StringMapWrapper.get(this._optionals, controlName);
@@ -331,8 +344,10 @@ export class ControlArray extends AbstractControl {
    */
   get length(): number { return this.controls.length; }
 
+  /** @internal */
   _updateValue(): void { this._value = this.controls.map((control) => control.value); }
 
+  /** @internal */
   _setParentForControls(): void {
     this.controls.forEach((control) => { control.setParent(this); });
   }