title: ScaffoldMessenger throws a hero animation error
body: <p>I am using the new ScaffoldMessenger to show a snackbar if a user successfully creates a project.
  While showing the snackbar, i navigate the app to the dashboard. But as soon as it hits the dashboard <strong>There are multiple heroes that share the same tag within a subtree</strong> error is thrown.
  I am not using any Hero widget in my dashbard and I have one FloatingActionButton but its hero parameter is set to null.</p>
  <p><strong>Sample code:</strong></p>
  <pre class="lang-dart prettyprint-override"><code>ScaffoldMessenger.of(context).showSnackBar(
          SnackBar(
            content: Text('A SnackBar has been shown.'),
            animation: null,
          ),
        );
  Navigator.pushReplacementNamed(context, '/dashboard');
  </code></pre>
  <p>Which results in this error:</p>
  <pre><code>The following assertion was thrown during a scheduler callback:
  There are multiple heroes that share the same tag within a subtree.
  
  Within each subtree for which heroes are to be animated (i.e. a PageRoute subtree), each Hero must have a unique non-null tag.
  In this case, multiple heroes had the following tag: &lt;SnackBar Hero tag - Text(&quot;A SnackBar has been shown.&quot;)&gt;
  
  Within each subtree for which heroes are to be animated (i.e. a PageRoute subtree), each Hero must have a unique non-null tag.
  In this case, multiple heroes had the following tag: &lt;SnackBar Hero tag - Text(&quot;A SnackBar has been shown.&quot;)&gt;
  
  Here is the subtree for one of the offending heroes: Hero
      tag: &lt;SnackBar Hero tag - Text(&quot;A SnackBar has been shown.&quot;)&gt;
      state: _HeroState#7589f
  When the exception was thrown, this was the stack
  #0      Hero._allHeroesFor.inviteHero.&lt;anonymous closure&gt;
  #1      Hero._allHeroesFor.inviteHero
  package:flutter/…/widgets/heroes.dart:277
  #2      Hero._allHeroesFor.visitor
  package:flutter/…/widgets/heroes.dart:296
  #3      ComponentElement.visitChildren
  package:flutter/…/widgets/framework.dart:4729
  #4      Hero._allHeroesFor.visitor
  package:flutter/…/widgets/heroes.dart:309
  ...
  
  
  </code></pre>