Add flag to track if a constructor is the synthetic default constructor.

pkg/kernel/lib/text/ast_to_text.dart

Index: pkg/kernel/lib/text/ast_to_text.dart
diff --git a/pkg/kernel/lib/text/ast_to_text.dart b/pkg/kernel/lib/text/ast_to_text.dart
index 198c21462d0cd9b6da7859e38e559d4603721602..eb2652af88ae8ed722445de6b78c3c0a26a2ba65 100644
--- a/pkg/kernel/lib/text/ast_to_text.dart
+++ b/pkg/kernel/lib/text/ast_to_text.dart
@@ -733,6 +733,7 @@ class Printer extends Visitor<Null> {
     writeIndentation();
     writeModifier(node.isExternal, 'external');
     writeModifier(node.isConst, 'const');
+    writeModifier(node.isSyntheticDefault, 'default');
     writeWord('constructor');
     writeFunction(node.function,
         name: node.name, initializers: node.initializers);