'NativeScriptDomAdapter.getUserAgent' implementation request #831

Hi,

this commit:
angular/angular@5efc860#diff-49c3adf9b728e2991a90a190be9844f8
for this PR:
angular/angular#15256
breaks the 'ngDefaultControl' directive, because 'NativeScriptDomAdapter.getUserAgent' throws 'Not implemented!':

stack trace: JS: "Error: Not implemented!\n at NativeScriptDomAdapter.getUserAgent (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/nativescript-angular/dom-adapter.js:144:73) [angular]\n at _isAndroid (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/forms/bundles/forms.umd.js:555:110) [angular]\n at new DefaultValueAccessor (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/forms/bundles/forms.umd.js:588:38) [angular]\n at createClass (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:10905:26) [angular]\n at createDirectiveInstance (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:10730:37) [angular]\n at createViewNodes (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:12093:49) [angular]\n at callViewAction (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:12537:13) [angular]\n at execComponentViewsAction (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:12446:13) [angular]\n at createViewNodes (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:12120:5) [angular]\n at createRootView (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:11998:5) [angular]\n at callWithDebugContext (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:13213:42) [angular]\n at Object.debugCreateRootView [as createRootView] (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:12673:12) [angular]\n at ComponentFactory_.create (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:9919:46) [angular]\n at ComponentFactoryBoundToModule.create (file:///data/data/org.nativescript.AngularDynaform/files/app/tns_modules/@angular/core/bundles/core.umd.js:3455:29) [angular]"

Guenter