Introduces "--initializing-formal-access".

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index ad689fd42d9c7609863c491d7ae0ec0726d69d23..8ddc535314d0be4a6cd7b9b724eafaebd7dd7c83 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -665,6 +665,7 @@ main() => new B();
       MessageKind.DUPLICATE_DEFINITION: const MessageTemplate(
           MessageKind.DUPLICATE_DEFINITION,
           "Duplicate definition of '#{name}'.",
+          options: const ["--initializing-formal-access"],
           howToFix: "Try to rename or remove this definition.",
           examples: const [
             """
@@ -676,7 +677,16 @@ class C {
 main() {
   new C();
 }
+""",
+            """
+class C {
+  int x;
+  C(this.x, int x);
+}
 
+main() {
+  new C(4, 2);
+}
 """
           ]),