How to extend FormGroup correctly

I'm currently trying to extend the FormGroup type of Angular5 to implement a custom method to receive modelstate errors and display them into another custom component.

Unfortunately, I've been receiving a not so informative error:
Error: ServerValidatedForm/src/platform/svalidated/group.component.ts:30:1: Error encountered in metadata generated for exported symbol 'SVFormGroup': ServerValidatedForm/src/platform/svalidated/group.component.ts:33:22: Metadata collected contains an error that will be reported at runtime: Expression form not supported.

I'm suspecting that there's something that I miss on TypeScripting itself. Its good to notice that I've already cleaned up node_modules, removed all metadata from project and transpiled again and, every time I hit the same error.

I'm using:
"@angular/animations": "5.0.1", "@angular/cli": "^1.5.0", "@angular/common": "5.0.1", "@angular/compiler": "^5.0.1", "@angular/compiler-cli": "^5.0.1", "@angular/core": "5.0.1", "@angular/forms": "^5.0.1"

The important code is here, and ANY help will be of assitance:

import { Injectable } from "@angular/core"; import { FormGroup } from "@angular/forms"; import { AbstractControlOptions } from "@angular/forms/src/model";

@Injectable()
export class SVFormGroup extends FormGroup {
  constructor(
    controls: {[key: string]: any},
    validatorOrOpts?: ValidatorFn|ValidatorFn[]|AbstractControlOptions|null,
    asyncValidator?: AsyncValidatorFn|AsyncValidatorFn[]|null) {
      super(controls, validatorOrOpts, asyncValidator);  
    }

setFromModelState(response: any, form: SVFormGroup) {
if(response.error && response.error.ModelState){
  let validationErrorDictionary = response.error.ModelState;
  for (var fieldName in validationErrorDictionary) {
    if (validationErrorDictionary.hasOwnProperty(fieldName)) {
      if (form.controls[fieldName]) {
        form.controls[fieldName].setErrors(validationErrorDictionary[fieldName]);
      } else {                          
        this.setErrors(Object.assign(this.errors, validationErrorDictionary[fieldName]));
      }
    }
  }
}

} } `

And the complete code can be found at: https://github.com/eduelias/servervalidatedform