Use HTMLImports.whenReady for detecting HTML import loading

When using the v1 polyfills, Polymer depends on the load event for some
internal bookkeeping. This means that we cannot start using an imported
web component if our load listener is registered before Polymer's load
listener. Instead, we can use the HTMLImports.whenReady callback since
it's fired only after Polymer has had a chance to register everything.

Fixes vaadin/flow-demo#203

hummingbird-client/src/main/java/com/vaadin/client/ResourceLoader.java

@@ -17,6 +17,7 @@
 package com.vaadin.client;
 
 import com.google.gwt.core.client.Duration;
+import com.google.gwt.core.client.GWT;
 import com.google.gwt.core.client.Scheduler;
 import com.google.gwt.core.client.Scheduler.RepeatingCommand;
 import com.google.gwt.user.client.Timer;
@@ -125,6 +126,9 @@ public interface ResourceLoadListener {
 
     private Registry registry;
 
+    private final boolean supportsHtmlWhenReady = GWT.isClient()
+            && supportsHtmlWhenReady();
+
     /**
      * Creates a new resource loader. You should not create you own resource
      * loader, but instead use {@link Registry#getResourceLoader()} to get an
@@ -275,21 +279,56 @@ public void loadHtml(final String htmlUrl,
             linkTag.setAttribute("rel", "import");
             linkTag.setAttribute("href", url);
 
-            addOnloadHandler(linkTag, new ResourceLoadListener() {
+            class LoadAndReadyListener
+                    implements ResourceLoadListener, Runnable {
+                private boolean errorFired = false;
+
+                @Override
+                public void run() {
+                    // Invoked through HTMLImports.whenReady
+                    if (!errorFired) {
+                        fireLoad(event);
+                    }
+                }
+
                 @Override
                 public void onLoad(ResourceLoadEvent event) {
-                    fireLoad(event);
+                    if (!supportsHtmlWhenReady) {
+                        assert !errorFired;
+                        fireLoad(event);
+                    }
                 }
 
                 @Override
                 public void onError(ResourceLoadEvent event) {
+                    assert !errorFired;
+                    errorFired = true;
                     fireError(event);
                 }
-            }, event);
+            }
+            LoadAndReadyListener listener = new LoadAndReadyListener();
+
+            addOnloadHandler(linkTag, listener, event);
             getHead().appendChild(linkTag);
+
+            if (supportsHtmlWhenReady) {
+                addHtmlImportsReadyHandler(listener);
+            }
         }
     }
 
+    private static native boolean supportsHtmlWhenReady()
+    /*-{
+        return !!($wnd.HTMLImports && $wnd.HTMLImports.whenReady);
+    }-*/;
+
+    private static native void addHtmlImportsReadyHandler(Runnable handler)
+    /*-{
+        $wnd.HTMLImports.whenReady($entry(function() {
+            handler.@Runnable::run()();
+        }));
+    }-*/;
+
     /**
      * Adds an onload listener to the given element, which should be a link or a
      * script tag. The listener is called whenever loading is complete or an