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.
+// TODO(sigmund): rename to CompliationMessage
 abstract class CompilationError {
-  /// 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;
 }