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

pkg/front_end/lib/src/simple_error.dart

 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);
+
+  String toString() => "FrontEnd Error: $message";
 }