Core: Provide an instance of `TranslateService` to `MissingTranslationHandler`

[DethAriel]

I'm submitting a ... (check one with "x")

[ ] bug report => check the FAQ and search github for a similar issue or PR before submitting
[ ] support request => check the FAQ and search github for a similar issue before submitting
[x] feature request

Current behavior
If I implement the MissingTranslationHandler like this:

@Injectable()
export class MyHandler implements MissingTranslationHandler {
  constructor(private translateService: TranslateService) {
  }

  public handle(key: string) {
    return this.translateService.get('MISSING', { original: key });
  }
}

then this results in a cyclic dependency: TranslateService depends on MissingTranslationHandler, which in turn depends on TranslateService.

Expected/desired behavior
The desired behavior would be if the handle method provided an instance of TranslateService, e.g.

public handle(key: string, translateService: TranslateService) {
  return translateService.get('MISSING', { original: key });
}

or, given the #214 proposal, something like

interface TranslateHandleArgs {
  key: string;
  translation: TranslateService;
  interpolateParams?: any;
}

public handle(params: TranslateHandleArgs) {
  return params.translation.get('MISSING', { original: key });
}

Such a thing, in turn, would potentially trigger an infinite loop if the 'MISSING' key is missing, too. This should be taken into account somehow.

What is the motivation / use case for changing the behavior?

Suppose you need to convert API error codes into localized messages. APIs change frequently, and if you haven't yet covered a specific error code, then you need to output something generic along the lines of "An error occurred, code: {{code}}". That is the case this feature request would cover.

Please tell us about your environment:

• ng2-translate version: 2.4.3
• Angular version: 2.0.0-rc.6
• Browser: all
• Language: TypeScript 1.8.10

[DethAriel]

Here's a workaround if someone needs it:

import { Injectable, Injector } from '@angular/core';
import { MissingTranslationHandler, TranslateService } from 'ng2-translate';

@Injectable()
export class CustomMissingTranslationHandler extends MissingTranslationHandler {
  private missingKey: string;
  private translation: TranslateService;
  constructor(private injector: Injector) {
    super();
  }
  public handle(key: string) {
    if (this.translation == null) {
      this.translation = this.injector.get(TranslateService);
    }

    if (this.missingKey != null) {
      // already handling the missing translation, and the '__TRANSLATION_MISSING' part is missing, too.
      // Return the key itself
      let previouslyMissingKey = this.missingKey;
      this.missingKey = null;
      return previouslyMissingKey;
    }

    this.missingKey = key;
    // do your thing using this.translation:
    let result = this.translation.instant('__TRANSLATION_MISSING', { key });
    this.missingKey = null;
    return result;
  }
}

[ocombe]

The missing translations handler needs more love, I would love a pull request improving it because I don't have much time to work on this