Resynthesize constructor initializers from Kernel.

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 5c432803f549d673c68e4be3251a29179143b1d6..56c1f4ce16c87eb62f8c6fd5d504dfadc26b0f7e 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -1254,6 +1254,10 @@ enum ProcedureKind {
 
 /// Part of an initializer list in a constructor.
 abstract class Initializer extends TreeNode {
+  /// True if this is a synthetic constructor initializer.
+  @informative
+  bool isSynthetic = false;
+
   accept(InitializerVisitor v);
 }