ngo is removing rxjs add operators

[StephenFluin]

Using:

@angular/cli: 1.3.0-rc.1
node: 8.1.3
os: linux x64
@angular/animations: 4.3.1
@angular/cli: 1.3.0-rc.1
@angular/common: 4.3.1
@angular/compiler: 4.3.1
@angular/compiler-cli: 4.3.1
@angular/core: 4.3.1
@angular/forms: 4.3.1
@angular/http: 4.3.1
@angular/material: 2.0.0-beta.2
@angular/platform-browser: 4.3.1
@angular/platform-browser-dynamic: 4.3.1
@angular/router: 4.3.1
@angular/service-worker: 1.0.0-beta.16

I have import 'rxjs/add/operator/filter'; at the top of my app.component.ts file.

Expected Behavior:
When I use .filter on an observable, I expect the call to succeed

Observed Behavior
The filter method is missing from the prototype of Observable.

Other Background
In the source code generated with ng build -prod -bo, the import is being marked as pure.

        /*@__PURE__*/
        __webpack_require__("gbhw");