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

mojo/dart/packages/_mojo_for_test_only/lib/math/math_calculator.mojom.dart

Index: mojo/dart/packages/_mojo_for_test_only/lib/math/math_calculator.mojom.dart
diff --git a/mojo/dart/packages/_mojo_for_test_only/lib/math/math_calculator.mojom.dart b/mojo/dart/packages/_mojo_for_test_only/lib/math/math_calculator.mojom.dart
index 52913ba3bb8771e75a34817a4be65c5ba3be6a37..d7d2fc602ee1255012954a466f37299a95eb6ead 100644
--- a/mojo/dart/packages/_mojo_for_test_only/lib/math/math_calculator.mojom.dart
+++ b/mojo/dart/packages/_mojo_for_test_only/lib/math/math_calculator.mojom.dart
@@ -642,14 +642,19 @@ class CalculatorProxy implements bindings.ProxyBase {
 
 
 class CalculatorStub extends bindings.Stub {
-  Calculator _impl = null;
+  Calculator _impl;
 
   CalculatorStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [Calculator impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  CalculatorStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  CalculatorStub.fromHandle(
+      core.MojoHandle handle, [Calculator impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   CalculatorStub.unbound() : super.unbound();
 
@@ -682,7 +687,9 @@ class CalculatorStub 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 _calculatorMethodClearName:
         var response = _impl.clear(_calculatorClearResponseParamsFactory);
@@ -757,10 +764,23 @@ class CalculatorStub extends bindings.Stub {
 
   Calculator get impl => _impl;
   set impl(Calculator 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 "CalculatorStub($superString)";
@@ -790,7 +810,7 @@ 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);