title: Botbuilder: prompt don't wait user input
body: <p>I'm learn botbuilder and need to make WaterfallDialog. Steps in WaterfallDialog works, but prompts don't wait for user input</p>
  <pre><code>async beginDialog(dc: DialogContext): Promise&lt;DialogTurnResult&lt;any&gt;&gt; {
          dc.dialogs.add(new WaterfallDialog('start', [
              async (step) =&gt; {
                  // Ask user their name
                  return await step.prompt('namePrompt', `What's your name?`, ['yes', 'no']);
              },
              async (step) =&gt; {
                  // Remember the users answer
                  step.values['name'] = step.result;
  
                  // Ask user their age.
                  return await step.prompt('agePrompt', `Hi ${step.values['name']}. How old are you?`);
              },
              async (step) =&gt; {
                  // Remember the users answer
                  step.values['age'] = step.result;
  
                  // End the component and return the completed profile.
                  return await step.endDialog(step.values);
              }
          ]));
  
          dc.dialogs.add(new TextPrompt('namePrompt'));
          dc.dialogs.add(new NumberPrompt('agePrompt'));
  
          return await dc.beginDialog(this.initialDialogId);
      }
  </code></pre>
  <p>`</p>