Produce error when duplicate field initializers are found

tests/language/constructor_duplicate_initializers_test.dart

Index: tests/language/constructor_duplicate_initializers_test.dart
diff --git a/tests/language/constructor_duplicate_initializers_test.dart b/tests/language/constructor_duplicate_initializers_test.dart
index ab9a0994fcd091d91787278deb5986d6f531d036..df64c5f3d198c97462bff14c0df3a3d12cee5a59 100644
--- a/tests/language/constructor_duplicate_initializers_test.dart
+++ b/tests/language/constructor_duplicate_initializers_test.dart
@@ -15,10 +15,17 @@ class Class {
       // formals and initializer list.
     : field_ = 2  /// 02: compile-time error
   ;
+  // Test against duplicate final field initialization in initializing formals.
+  Class.two_fields(this.field_
+    , this.field_ /// 03: compile-time error
+  );
   final field_;
 }
 
 main() {
   new Class(42);
   new Class.field(42);
+  new Class.two_fields(42
+    , 42   /// 03: continued
+  );
 }