Tweak public APIs and use them in patch_sdk, dart2js, and kernel-service

pkg/front_end/lib/src/simple_error.dart

Index: pkg/front_end/lib/src/simple_error.dart
diff --git a/pkg/front_end/lib/src/simple_error.dart b/pkg/front_end/lib/src/simple_error.dart
deleted file mode 100644
index 40e0a4183eb4743e841ee07f8e2a3f056e3ddd4e..0000000000000000000000000000000000000000
--- a/pkg/front_end/lib/src/simple_error.dart
+++ /dev/null
@@ -1,11 +0,0 @@
-import 'package:front_end/compilation_error.dart';
-
-import 'package:source_span/source_span.dart' show SourceSpan;
-
-/// An error that only contains a message and no error location.
-class SimpleError implements CompilationError {
-  String get correction => null;
-  SourceSpan get span => null;
-  final String message;
-  SimpleError(this.message);
-}