Revert "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
new file mode 100644
index 0000000000000000000000000000000000000000..40e0a4183eb4743e841ee07f8e2a3f056e3ddd4e
--- /dev/null
+++ b/pkg/front_end/lib/src/simple_error.dart
@@ -0,0 +1,11 @@
+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);
+}