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

mojo/dart/packages/_mojo_for_test_only/lib/mojo/examples/echo.mojom.dart

Index: mojo/dart/packages/_mojo_for_test_only/lib/mojo/examples/echo.mojom.dart
diff --git a/mojo/dart/packages/_mojo_for_test_only/lib/mojo/examples/echo.mojom.dart b/mojo/dart/packages/_mojo_for_test_only/lib/mojo/examples/echo.mojom.dart
index 2a132c8e6416ae6c3ca081c0963c52dc3dcef52b..c120ffd2d5bfd42a5752fb0945d4fa6b2851974e 100644
--- a/mojo/dart/packages/_mojo_for_test_only/lib/mojo/examples/echo.mojom.dart
+++ b/mojo/dart/packages/_mojo_for_test_only/lib/mojo/examples/echo.mojom.dart
@@ -310,14 +310,19 @@ class EchoProxy implements bindings.ProxyBase {
 
 
 class EchoStub extends bindings.Stub {
-  Echo _impl = null;
+  Echo _impl;
 
   EchoStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [Echo impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  EchoStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  EchoStub.fromHandle(
+      core.MojoHandle handle, [Echo impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   EchoStub.unbound() : super.unbound();
 
@@ -340,7 +345,9 @@ class EchoStub 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 _echoMethodEchoStringName:
         var params = _EchoEchoStringParams.deserialize(
@@ -373,10 +380,23 @@ class EchoStub extends bindings.Stub {
 
   Echo get impl => _impl;
   set impl(Echo 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 "EchoStub($superString)";
@@ -406,7 +426,7 @@ mojom_types.RuntimeTypeInfo  _initRuntimeTypeInfo() {
   // serializedRuntimeTypeInfo contains the bytes of the Mojo serialization of
   // a mojom_types.RuntimeTypeInfo struct describing the Mojom types in this
   // file. The string contains the base64 encoding of the gzip-compressed bytes.
-  var serializedRuntimeTypeInfo = "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";
+  var serializedRuntimeTypeInfo = "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";
 
   // Deserialize RuntimeTypeInfo
   var bytes = BASE64.decode(serializedRuntimeTypeInfo);