Detect if no JavaScript is build for specific browser in GWT

There are different ways to solve this problem, but the usual gwt way is using Deferred-Binding

Just create a class for loading the app code and instantiate it using GWT.create() in your onModuleLoad. Then you can have different implementations of the class for each permutation.

  package mynamespace.client;
  ...

  public class MyEntryPoint implements EntryPoint {

    public void onModuleLoad() {
      // create the appropriate implementation of the App class
      App app = GWT.create(App.class);
      // call the method to load the application.
      app.onLoad();
    }

    // Default implementation
    public static class App {
      public void onLoad() {
        Window.alert("This App is only supported in IE");
      }
    }

    // Implementation for IE
    public static class AppIE extends App {
      public void onLoad() {
        Window.alert("This is a supported Browser");
      }
    }
  }

Define which implementation use per each user-agent in your .gwt.xml file:

  <replace-with class="mynamespace.client.MyEntryPoint.AppIE">
    <when-type-is class="mynamespace.client.MyEntryPoint.App"/>
    <any>
         <when-property-is name="user.agent" value="ie6"/>
         <when-property-is name="user.agent" value="ie8"/>
         <when-property-is name="user.agent" value="ie9"/>
    </any>
  </replace-with>

IMO, I think the nocache.js bootstrap script should have a mechanism to alert the user when there is no permutation for a specific browser. Maybe calling the gwt:onLoadErrorFn (see my comment at gwt issue#8135)