Random improvements in error reporting.

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 'compiler_context.dart' show CompilerContext;
 
 import 'messages.dart'
     show
@@ skipping 12 matching lines @@
 /// and contain a short description that may help a developer debug the issue.
 /// This method should be called instead of using `throw`, as this allows us to
 /// ensure that there are no throws anywhere else in the codebase.
 ///
 /// Before printing the message, the string `"Internal error: "` is prepended.
 dynamic internalProblem(Message message, int charOffset, Uri uri) {
   throw CompilerContext.current
       .format(message.withLocation(uri, charOffset), Severity.internalProblem);
 }
 
-dynamic unimplemented(String what, [int charOffset = -1, Uri uri = null]) {
+dynamic unimplemented(String what, int charOffset, Uri uri) {
   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);
 }