Use messages for (some) public API errors

pkg/front_end/lib/src/fasta/problems.dart

 // Copyright (c) 2017, 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.
 
 library fasta.problems;
 
 import 'messages.dart'
     show
         Message,
         deprecated_format,
@@ skipping 16 matching lines @@
   if (message.tip != null) {
     text += "\n${message.tip}";
   }
   if (uri == null && charOffset == -1) {
     throw text;
   } else {
     throw deprecated_format(uri, charOffset, text);
   }
 }
 
-dynamic unimplemented(String what, int charOffset, Uri uri) {
+dynamic unimplemented(String what, [int charOffset = -1, Uri uri = null]) {
   return internalProblem(
       templateInternalProblemUnimplemented.withArguments(what),
       charOffset,
       uri);
 }
 
 dynamic unhandled(String what, String where, int charOffset, Uri uri) {
   return internalProblem(
       templateInternalProblemUnhandled.withArguments(what, where),
       charOffset,
       uri);
 }
 
 dynamic unexpected(String expected, String actual, int charOffset, Uri uri) {
   return internalProblem(
       templateInternalProblemUnexpected.withArguments(expected, actual),
       charOffset,
       uri);
 }
 
 dynamic unsupported(String operation, int charOffset, Uri uri) {
   return internalProblem(
       templateInternalProblemUnsupported.withArguments(operation),
       charOffset,
       uri);
 }