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
index 40e0a4183eb4743e841ee07f8e2a3f056e3ddd4e..fd11ccd678edb531bcc94aac223dbc8a05bb412e 100644
--- a/pkg/front_end/lib/src/simple_error.dart
+++ b/pkg/front_end/lib/src/simple_error.dart
@@ -8,4 +8,6 @@ class SimpleError implements CompilationError {
   SourceSpan get span => null;
   final String message;
   SimpleError(this.message);
+
+  String toString() => "FrontEnd Error: $message";

[ahe, 2017/07/05 13:29:40]

I'm not sure how the user experience is improved by adding "FrontEnd Error: " to
the message.

[Siggi Cherem (dart-lang), 2017/07/05 18:42:11]

Done.

 }