Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/source/stack_listener.dart

Index: pkg/front_end/lib/src/fasta/source/stack_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/stack_listener.dart b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
index 0e03ae66ec42dd0d9141cf5eae139191bcf978ce..7b5805499ff8b3b52e8dbe2a25bb8ae7df6fe5ba 100644
--- a/pkg/front_end/lib/src/fasta/source/stack_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
@@ -55,7 +55,8 @@ abstract class StackListener extends Listener {
 
   // TODO(ahe): This doesn't belong here. Only implemented by body_builder.dart
   // and ast_builder.dart.
-  void finishFunction(formals, AsyncMarker asyncModifier, body) {
+  void finishFunction(
+      covariant formals, AsyncMarker asyncModifier, covariant body) {

[ahe, 2017/02/07 11:03:08]

Would dynamic work instead of covariant?

[Siggi Cherem (dart-lang), 2017/02/07 21:59:26]

not in this case: these were treated as dynamic, the problem is that there are 2
overrides that strenghten the types:
- analyzer/AstBuilder makes it dynamic x AsyncMarker x analyzer.Statement
- kernel/BodyBuilder makes it FormalParameters x AsyncMarker x kernel.Statement

I could however move the covariant down to each override if you prefer

[ahe, 2017/02/08 19:59:24]

Sigh. Obviously someone doesn't know what "dynamic" means. So long, optional
types :-((((

     return internalError("Unsupported operation");
   }