Add optional message to assert in Dart2js.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index 8ad289f9d38028b1ea429755a024a6f78450792b..8896bf455feedf9c16b3384299983e9ca7b83d62 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -650,20 +650,6 @@ abstract class IrBuilderVisitor extends ast.Visitor<ir.Primitive>
 
   // ## Sends ##
   @override
-  ir.Primitive visitAssert(ast.Send node, ast.Node condition, _) {
-    assert(irBuilder.isOpen);
-    if (compiler.enableUserAssertions) {
-      return giveup(node, 'assert in checked mode not implemented');
-    } else {
-      // The call to assert and its argument expression must be ignored
-      // in production mode.
-      // Assertions can only occur in expression statements, so no value needs
-      // to be returned.
-      return null;
-    }
-  }
-
-  @override
   void previsitDeferredAccess(ast.Send node, PrefixElement prefix, _) {
     giveup(node, 'deferred access is not implemented');
   }