Add a "correction" field to front end compilation errors.

pkg/front_end/lib/compilation_error.dart

Index: pkg/front_end/lib/compilation_error.dart
diff --git a/pkg/front_end/lib/compilation_error.dart b/pkg/front_end/lib/compilation_error.dart
index 2211ae98c2237e5b9c5f76568047e71f9f362a0c..a2385759482cb147c1b7e8061fee76fd03712f28 100644
--- a/pkg/front_end/lib/compilation_error.dart
+++ b/pkg/front_end/lib/compilation_error.dart
@@ -13,14 +13,14 @@ import 'package:source_span/source_span.dart' show SourceSpan;
 ///
 /// TODO(paulberry): add a reference to the analyzer error code.
 ///
-/// TODO(paulberry): add a correction message, once most analyzer errors have
-/// one.
-///
 /// Not intended to be implemented or extended by clients.
 abstract class CompilationError {
-  /// A text description of the compile error.
-  String get message;
+  /// A text description of how the user can fix the error.  May be `null`.
+  String get correction;
 
   /// The source location where the error occurred.
   SourceSpan get location;
+
+  /// A text description of the compile error.
+  String get message;
 }