title: ngForm crash in IE 11 for multiple scenario
body: <p>ngForm crash in IE 11 for multiple scenario.</p>
  
  <ol>
  <li>after page load if user directly click 'Submit' button without touching any textbox then browser getting refresh.</li>
  <li>If user remove focus from 'username' textbox by clicking somewhere else on form and then click 'submit' button then Validation Msg displays,
  After validation msg display if user enter any value on 'Password' textbox and try to remove it browser getting refresh.</li>
  </ol>
  
  <p>Other scenario is woring on other browser but getting crash on IE only.</p>
  
  <p>This is happening because of '<strong>is-invalid</strong>' class. I tried multiple code changes but did not work for IE</p>
  
  <pre><code>&lt;form class="login" name="form" #f="ngForm" novalidate&gt;
          &lt;input type="text" placeholder="Username" autofocus #userName class="form-control" name="username" [(ngModel)]="user.username" #username="ngModel" 
          [ngClass]="{ 'is-invalid': (f.submitted &amp;&amp; username.invalid ) ? true : false }"
           required autocomplete="off"
          /&gt;
          &lt;i class="fa fa-user"&gt;&lt;/i&gt;
          &lt;div *ngIf="f.submitted &amp;&amp; username.invalid" class="invalid-feedback"&gt;
              &lt;div *ngIf="username.errors.required"&gt;Username is required&lt;/div&gt;
          &lt;/div&gt;
          &lt;input type="password" placeholder="Password" class="form-control" name="password" [(ngModel)]="user.password" #password="ngModel" 
          [ngClass]="{ 'is-invalid': (f.submitted &amp;&amp; password.invalid) ? true : false}"
           required /&gt;
          &lt;i class="fa fa-key"&gt;&lt;/i&gt;
          &lt;div *ngIf="f.submitted &amp;&amp; password.invalid" class="invalid-feedback"&gt;
              &lt;div *ngIf="password.errors.required"&gt;Password is required&lt;/div&gt;
          &lt;/div&gt;
          &lt;div *ngIf="loginFailed" class="invalid-feedback"&gt;
              &lt;div *ngIf="loginFailed"&gt;{{loginFailedMsg}}&lt;/div&gt;
          &lt;/div&gt;
          &lt;button type="submit" (click)="login()"&gt; &lt;span class="state"&gt;Log in&lt;/span&gt;&lt;/button&gt;
      &lt;/form&gt;```
  </code></pre>