How to set a form as pristine?

There is the markAsPristine method (it seems undocumented for now, but can be found here: https://github.com/angular/angular/blob/53f0c2206df6a5f8ee03d611a7563ca1a78cc82d/tools/public_api_guard/forms/index.d.ts#L42).

Basically, this.form.markAsPristine() does what you expect.

update

In the new forms module this was improved a lot.

AbstractControl, the base class of most form classes provides

markAsTouched({onlySelf}?: {onlySelf?: boolean}) : void
markAsUntouched({onlySelf}?: {onlySelf?: boolean}) : void
markAsDirty({onlySelf}?: {onlySelf?: boolean}) : void
markAsPristine({onlySelf}?: {onlySelf?: boolean}) : void
markAsPending({onlySelf}?: {onlySelf?: boolean}) : void

And several other new methods

disable({onlySelf, emitEvent}?: {onlySelf?: boolean, emitEvent?: boolean}) : void
enable({onlySelf, emitEvent}?: {onlySelf?: boolean, emitEvent?: boolean}) : void

setValue(value: any, options?: Object) : void
patchValue(value: any, options?: Object) : void

reset(value?: any, options?: Object) : void
updateValueAndValidity({onlySelf, emitEvent}?: {onlySelf?: boolean,  emitEvent?: boolean}) : void // (old)
setErrors(errors: {[key: string]: any}, {emitEvent}?: {emitEvent?: boolean}) : void

original

That's currently not supported. See https://github.com/angular/angular/issues/5568 and https://github.com/angular/angular/issues/4933. The usual workaround is to re-create the form to get a pristine one.