All strong mode cleaning of dart2js.

pkg/compiler/lib/src/js_backend/namer.dart

Index: pkg/compiler/lib/src/js_backend/namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index 15751cfe3a2151b13cfdc64b33968b6d889e757c..4b109ed853eeb8fdcf1afe783cb46412bd51fadb 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -1884,7 +1884,7 @@ class ConstantNamingVisitor implements ConstantValueVisitor {
   }
 
   @override
-  void visitMap(JavaScriptMapConstant constant, [_]) {
+  void visitMap(covariant JavaScriptMapConstant constant, [_]) {
     // TODO(9476): Incorporate type parameters into name.
     addRoot('Map');
     if (constant.length == 0) {
@@ -2178,15 +2178,15 @@ class FunctionTypeNamer extends BaseResolutionDartTypeVisitor {
     return sb.toString();
   }
 
-  visit(ResolutionDartType type, [_]) {
+  visit(covariant ResolutionDartType type, [_]) {
     type.accept(this, null);
   }
 
-  visitType(ResolutionDartType type, _) {
+  visitType(covariant ResolutionDartType type, _) {
     sb.write(type.name);
   }
 
-  visitFunctionType(ResolutionFunctionType type, _) {
+  visitFunctionType(covariant ResolutionFunctionType type, _) {
     if (rtiEncoder.isSimpleFunctionType(type)) {
       sb.write('args${type.parameterTypes.length}');
       return;