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

mojo/dart/packages/mojo_services/lib/mojo/ui/view_containers.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/ui/view_containers.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/ui/view_containers.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/ui/view_containers.mojom.dart
index 5b72e9d1bfe92faeacece143897195e3846e16a0..40db90eee2d3ab7a3b12ad574a40a14ed00ad9b1 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/ui/view_containers.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/ui/view_containers.mojom.dart
@@ -866,14 +866,19 @@ class ViewContainerProxy implements bindings.ProxyBase {
 
 
 class ViewContainerStub extends bindings.Stub {
-  ViewContainer _impl = null;
+  ViewContainer _impl;
 
   ViewContainerStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [ViewContainer impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  ViewContainerStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  ViewContainerStub.fromHandle(
+      core.MojoHandle handle, [ViewContainer impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   ViewContainerStub.unbound() : super.unbound();
 
@@ -891,7 +896,9 @@ class ViewContainerStub 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 _viewContainerMethodSetListenerName:
         var params = _ViewContainerSetListenerParams.deserialize(
@@ -922,10 +929,23 @@ class ViewContainerStub extends bindings.Stub {
 
   ViewContainer get impl => _impl;
   set impl(ViewContainer 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 "ViewContainerStub($superString)";
@@ -1123,14 +1143,19 @@ class ViewContainerListenerProxy implements bindings.ProxyBase {
 
 
 class ViewContainerListenerStub extends bindings.Stub {
-  ViewContainerListener _impl = null;
+  ViewContainerListener _impl;
 
   ViewContainerListenerStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [ViewContainerListener impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  ViewContainerListenerStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  ViewContainerListenerStub.fromHandle(
+      core.MojoHandle handle, [ViewContainerListener impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   ViewContainerListenerStub.unbound() : super.unbound();
 
@@ -1156,7 +1181,9 @@ class ViewContainerListenerStub 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 _viewContainerListenerMethodOnChildAttachedName:
         var params = _ViewContainerListenerOnChildAttachedParams.deserialize(
@@ -1211,10 +1238,23 @@ class ViewContainerListenerStub extends bindings.Stub {
 
   ViewContainerListener get impl => _impl;
   set impl(ViewContainerListener 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 "ViewContainerListenerStub($superString)";