Add optional message to assert in Dart2js.

pkg/compiler/lib/src/resolution/semantic_visitor_mixins.dart

Index: pkg/compiler/lib/src/resolution/semantic_visitor_mixins.dart
diff --git a/pkg/compiler/lib/src/resolution/semantic_visitor_mixins.dart b/pkg/compiler/lib/src/resolution/semantic_visitor_mixins.dart
index fce11790003dacfdbad320ee273c2e431740c833..5e15a7dcd36af84a8d024e69fe8af6d73f6f2fbb 100644
--- a/pkg/compiler/lib/src/resolution/semantic_visitor_mixins.dart
+++ b/pkg/compiler/lib/src/resolution/semantic_visitor_mixins.dart
@@ -27,14 +27,6 @@ abstract class ErrorBulkMixin<R, A>
   }
 
   @override
-  R errorInvalidAssert(
-      Send node,
-      NodeList arguments,
-      A arg) {
-    return bulkHandleError(node, null, arg);
-  }
-
-  @override
   R errorNonConstantConstructorInvoke(
       NewExpression node,
       Element element,
@@ -2943,14 +2935,6 @@ abstract class BaseBulkMixin<R, A>
   }
 
   @override
-  R visitAssert(
-      Send node,
-      Node expression,
-      A arg) {
-    return bulkHandleNode(node, 'Assert `#` unhandled.', arg);
-  }
-
-  @override
   R visitIs(
       Send node,
       Node expression,
@@ -4535,15 +4519,6 @@ class TraversalSendMixin<R, A> implements SemanticSendVisitor<R, A> {
   }
 
   @override
-  R visitAssert(
-      Send node,
-      Node expression,
-      A arg) {
-    apply(expression, arg);
-    return null;
-  }
-
-  @override
   R visitBinary(
       Send node,
       Node left,