[kernel] Fix assert message.

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index eb988a582c60476921b12ad5dee5ed133606e031..dcc0285ef761ae0f0e3c3e146f4b52626fbc384f 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -3161,8 +3161,11 @@ class EmptyStatement extends Statement {
 class AssertStatement extends Statement {
   Expression condition;
   Expression message; // May be null.
+  int conditionStartOffset;
+  int conditionEndOffset;
 
-  AssertStatement(this.condition, [this.message]) {
+  AssertStatement(this.condition,
+      {this.message, this.conditionStartOffset, this.conditionEndOffset}) {
     condition?.parent = this;
     message?.parent = this;
   }