Remove deprecated_internalProblem.

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,
         templateUnexpected,
         templateUnhandled,
         templateUnimplemented,
         templateUnsupported;
 
 /// Used to report an internal error.
 ///
 /// Internal errors should be avoided as best as possible, but are preferred
-/// over assertion failures. The message should 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.
+/// over assertion failures. The message should start with "Internal error:"
+/// 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.
 dynamic internalProblem(Message message, int charOffset, Uri uri) {
-  String text = "Internal problem: ${message.message}";
   if (uri == null && charOffset == -1) {
-    throw text;
+    throw message.message;
   } else {
-    throw deprecated_format(uri, charOffset, text);
+    throw deprecated_format(uri, charOffset, message.message);
   }
 }
 
 dynamic unimplemented(String what, int charOffset, Uri uri) {
   return internalProblem(
       templateUnimplemented.withArguments(what), charOffset, uri);
 }
 
 dynamic unhandled(String what, String where, int charOffset, Uri uri) {
   return internalProblem(
       templateUnhandled.withArguments(what, where), charOffset, uri);
 }
 
 dynamic unexpected(String expected, String actual, int charOffset, Uri uri) {
   return internalProblem(
       templateUnexpected.withArguments(expected, actual), charOffset, uri);
 }
 
 dynamic unsupported(String operation, int charOffset, Uri uri) {
   return internalProblem(
       templateUnsupported.withArguments(operation), charOffset, uri);
 }