Add optional message to assert in Dart2js.

pkg/compiler/lib/src/inferrer/inferrer_visitor.dart

Index: pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
diff --git a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
index e542d49d2945624e2b259de679d8abb5409b6096..4f7603c7cb8a9f60e1ea78ac73f17181ba23533a 100644
--- a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
+++ b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
@@ -793,17 +793,6 @@ abstract class InferrerVisitor<T, E extends MinimalInferrerEngine<T>>
     return types.dynamicType;
   }
 
-  @override
-  T visitAssert(Send node, Node expression, _) {
-    if (!compiler.enableUserAssertions) {
-      return types.nullType;
-    }
-    return handleAssert(node, expression);
-  }
-
-  /// Handle an enabled assertion of [expression].
-  T handleAssert(Send node, Node expression);
-
   T visitNode(Node node) {
     return node.visitChildren(this);
   }