Dart: Wait to handle events on a Stub until it makes sense to do it.

mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart b/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
index 429e556b3ab8fa2352d096ed29b29e6ded7798f7..972231de903b65126e1a7c4e38605970cfa7adfe 100644
--- a/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
@@ -1039,14 +1039,19 @@ class AuthenticationServiceProxy implements bindings.ProxyBase {
 
 
 class AuthenticationServiceStub extends bindings.Stub {
-  AuthenticationService _impl = null;
+  AuthenticationService _impl;
 
   AuthenticationServiceStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [AuthenticationService impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  AuthenticationServiceStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  AuthenticationServiceStub.fromHandle(
+      core.MojoHandle handle, [AuthenticationService impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   AuthenticationServiceStub.unbound() : super.unbound();
 
@@ -1090,7 +1095,9 @@ class AuthenticationServiceStub extends bindings.Stub {
                                                           0,
                                                           message);
     }
-    assert(_impl != null);
+    if (_impl == null) {
+      throw new core.MojoApiError("$this has no implementation set");
+    }
     switch (message.header.type) {
       case _authenticationServiceMethodSelectAccountName:
         var params = _AuthenticationServiceSelectAccountParams.deserialize(
@@ -1194,10 +1201,23 @@ class AuthenticationServiceStub extends bindings.Stub {
 
   AuthenticationService get impl => _impl;
   set impl(AuthenticationService d) {
-    assert(_impl == null);
+    if (d == null) {
+      throw new core.MojoApiError("$this: Cannot set a null implementation");
+    }
+    if (isBound && (_impl == null)) {
+      beginHandlingEvents();
+    }
     _impl = d;
   }
 
+  @override
+  void bind(core.MojoMessagePipeEndpoint endpoint) {
+    super.bind(endpoint);
+    if (!isOpen && (_impl != null)) {
+      beginHandlingEvents();
+    }
+  }
+
   String toString() {
     var superString = super.toString();
     return "AuthenticationServiceStub($superString)";