Use messages for (some) public API errors

pkg/front_end/lib/compilation_error.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Defines the API for the front end to communicate information about
 /// compilation errors to clients.
 library front_end.compilation_error;
 
 import 'package:source_span/source_span.dart' show SourceSpan;
 
 /// A single error that occurred during compilation, and information about where
-/// it occurred.
+/// it occurred and how to fix it.
 ///
 /// TODO(paulberry): add a reference to the analyzer error code.
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class CompilationError {

[Siggi Cherem (dart-lang), 2017/07/11 03:57:13]

Paul - I like the idea of renaming this to CompilationMessage or something like
that. The reason: we will report other messages that are not errors (warnings,
hints, etc).

[Paul Berry, 2017/07/11 16:00:59]

Fine by me :)

[ahe, 2017/07/12 13:14:43]

CompilationProblem?

[Siggi Cherem (dart-lang), 2017/07/12 21:54:58]

Is it always a problem? Don't we want the compiler sometimes to emit encouraging
messages like "Good job, your code looks really neat!" ;-)?

"problem" works for me too

[ahe, 2017/07/12 23:44:56]

CompilationOpportunity? ;-)

[ahe, 2017/07/13 00:06:03]

Comportunity? ;-)

-  /// A text description of how the user can fix the error.  May be `null`.
-  String get correction;
+  /// A text description of the compile error.
+  String get message;
+
+  /// A suggestion for the user to hint them on how to fix the error. May be
+  /// `null`.
+  String get tip;
 
   /// The source span where the error occurred.
   SourceSpan get span;
-
-  /// A text description of the compile error.
-  String get message;
 }