Remove deprecation from setTag and throw on subsequent calls

client/src/main/java/com/vaadin/client/ServerConnector.java

@@ -221,19 +221,20 @@ public interface ServerConnector extends Connector {
      * Sets the connector type tag for this connector. This should only be
      * called from
      * {@link WidgetSet#createConnector(int, ApplicationConfiguration)}
+     * <p>
+     * <strong>Note:</strong> This method is intended for internal use only.
      *
      * @see #getTag()
      *
      * @param tag
      *            the connector type tag
      *
-     * @deprecated This is an internal method and should not be called by an
-     *             application developer.
+     * @throws IllegalStateException
+     *             if {@code tag} has already been set
      *
      * @since 8.1
      */
-    @Deprecated
-    public void setTag(int tag);
+    public void setTag(int tag) throws IllegalStateException;
 
     /**
      * Gets the connector type tag for this connector. This type tag is an

client/src/main/java/com/vaadin/client/ui/AbstractConnector.java

@@ -61,7 +61,7 @@ public abstract class AbstractConnector
 
     private ApplicationConnection connection;
     private String id;
-    private int tag;
+    private int tag = -1;
 
     private HandlerManager handlerManager;
     private FastStringMap<HandlerManager> statePropertyHandlerManagers;
@@ -532,6 +532,10 @@ public abstract class AbstractConnector
 
     @Override
     public void setTag(int tag) {
+        if (this.tag >= 0) {
+            throw new IllegalStateException(
+                    "Tag already set for this " + getClass().getSimpleName());
+        }
         this.tag = tag;
     }
 }