Error when adding a handler if a channel has been created

The RPC class design requires all the handlers to be provided up-front.
Currently, adding a new handler after a channel is created has no
effect. To make more obvious this misuse, the method now throws an
error.

src/shared/bridge.js

@@ -143,8 +143,15 @@ export default class Bridge {
    *   callback of the type: `(error: string|Error|null, ...result: any[]) => void`.
    *   This callback must be invoked in order to respond (via `postMessage`)
    *   to the other frame/s with a result or an error.
+   * @throws {Error} If trying to register a callback after a channel has already been created
+   * @throws {Error} If trying to register a callback with the same name multiple times
    */
   on(method, callback) {
+    if (this.links.length > 0) {
+      throw new Error(
+        `Listener '${method}' can't be registered because a channel has already been created`
+      );
+    }
     if (this.channelListeners[method]) {
       throw new Error(`Listener '${method}' already bound in Bridge`);
     }

src/shared/test/bridge-test.js

@@ -168,14 +168,39 @@ describe('shared/bridge', () => {
 
   describe('#on', () => {
     it('adds a method to the method registry', () => {
-      createChannel();
       bridge.on('message1', sandbox.spy());
+      createChannel();
       assert.isFunction(bridge.channelListeners.message1);
     });
 
+    it('raise an error if trying to register a listener after a channel has been already created', () => {
+      createChannel();
+      let error;
+      try {
+        bridge.on('message1', () => {});
+      } catch (err) {
+        error = err;
+      }
+
+      assert.equal(
+        error.message,
+        "Listener 'message1' can't be registered because a channel has already been created"
+      );
+    });
+
     it('only allows registering a method once', () => {
-      bridge.on('message1', sandbox.spy());
-      assert.throws(() => bridge.on('message1', sandbox.spy()));
+      bridge.on('message1', () => {});
+      let error;
+      try {
+        bridge.on('message1', () => {});
+      } catch (err) {
+        error = err;
+      }
+
+      assert.equal(
+        error.message,
+        "Listener 'message1' already bound in Bridge"
+      );
     });
   });