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

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

Index: mojo/dart/packages/mojo_services/lib/mojo/tcp_bound_socket.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/tcp_bound_socket.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/tcp_bound_socket.mojom.dart
index d28f1ce88b29b41e8262cf5146f128a64bcd9aa6..5af9453ad23b24709851b9796bda2ca6f9ef3fda 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/tcp_bound_socket.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/tcp_bound_socket.mojom.dart
@@ -524,14 +524,19 @@ class TcpBoundSocketProxy implements bindings.ProxyBase {
 
 
 class TcpBoundSocketStub extends bindings.Stub {
-  TcpBoundSocket _impl = null;
+  TcpBoundSocket _impl;
 
   TcpBoundSocketStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [TcpBoundSocket impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  TcpBoundSocketStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  TcpBoundSocketStub.fromHandle(
+      core.MojoHandle handle, [TcpBoundSocket impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   TcpBoundSocketStub.unbound() : super.unbound();
 
@@ -559,7 +564,9 @@ class TcpBoundSocketStub 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 _tcpBoundSocketMethodStartListeningName:
         var params = _TcpBoundSocketStartListeningParams.deserialize(
@@ -614,10 +621,23 @@ class TcpBoundSocketStub extends bindings.Stub {
 
   TcpBoundSocket get impl => _impl;
   set impl(TcpBoundSocket 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 "TcpBoundSocketStub($superString)";