title: class variable is undefined after subscribing observable in lifecycle hook
body: <p>I try to set a variable (json object) in my component class. ngOnInit subscribing an observable service and set the component variable. 
  When I try to access this variable with dot notation in components template, I get this error:</p>
  
  <blockquote>
    <p>Cannot read property 'count_runs' of undefined</p>
  </blockquote>
  
  <p>The observable has a type annotation (avgTime Interface).</p>
  
  <p>avg-time.ts</p>
  
  <pre><code>export interface AvgTime {
    avg_runtime_millis: number;
    count_runs: number;
  }
  </code></pre>
  
  <p>stats.service.ts</p>
  
  <pre><code>import { Injectable } from '@angular/core';
  import { Http, Response } from '@angular/http';
  import { AvgTime } from './avg-time';
  import { Observable } from 'rxjs/Observable';
  
  @Injectable()
  export class StatsService {
    constructor(private _http: Http) { }
  
    private _avgTimeUrl = 'http://localhost:3002/api/reservation/avgTime';
  
    getAvgTime() : Observable&lt;AvgTime&gt; {
      return this._http.get(this._avgTimeUrl)
                 .map(res =&gt; res.json());
    }
  }
  </code></pre>
  
  <p>avg-time.component.ts</p>
  
  <pre><code>import {Component, OnInit} from '@angular/core';
  import { AvgTime } from './avg-time';
  import { StatsService } from './stats.service';
  
  @Component({
    selector: 'avg-time',
    template: `
      &lt;h1&gt;AvgTimeComponent&lt;/h1&gt;
      {{avgTime.count_runs}}
    `,
    providers: [StatsService]
  })
  export class AvgTimeComponent implements OnInit {
    avgTime: AvgTime;
  
    constructor(private _statsService: StatsService) { }
  
    ngOnInit() {
      this._statsService.getAvgTime()
          .subscribe(avgTime =&gt; this.avgTime = avgTime);
    }
  }
  </code></pre>
  
  <p>It is also not working when I fake the service response in ngOnInit like:</p>
  
  <p>avg-time.component.ts</p>
  
  <pre><code>  ngOnInit() {
      this._statsService.getAvgTime()
          .subscribe(avgTime =&gt; {
            this.avgTime = {
              avg_runtime_millis: 150,
              count_runs: 20
            };
          });
    }
  </code></pre>
  
  <p>The return of my stats.service.ts from backend is:</p>
  
  <pre><code>[{"AVG_RUNTIME_MILLIS":55,"COUNT_RUNS":5400}]
  </code></pre>