Deprecate all diagnostics methods that use strings.

pkg/front_end/lib/src/fasta/modifier.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.
 
 library fasta.modifier;
 
-import 'errors.dart' show internalError;
+import 'deprecated_problems.dart' show deprecated_internalProblem;
 
 enum ModifierEnum {
   Abstract,
   Const,
   Covariant,
   External,
   Final,
   Static,
 
   // Not a real modifier.
@@ skipping 42 matching lines @@
   const Modifier(this.kind, this.mask);
 
   factory Modifier.fromString(String string) {
     if (identical('abstract', string)) return Abstract;
     if (identical('const', string)) return Const;
     if (identical('covariant', string)) return Covariant;
     if (identical('external', string)) return External;
     if (identical('final', string)) return Final;
     if (identical('static', string)) return Static;
     if (identical('var', string)) return Var;
-    return internalError("Unhandled modifier: $string");
+    return deprecated_internalProblem("Unhandled modifier: $string");
   }
 
   toString() => "modifier(${'$kind'.substring('ModifierEnum.'.length)})";
 
   static int validate(List<Modifier> modifiers, {bool isAbstract: false}) {
     // TODO(ahe): Rename this method, validation is now taken care of by the
     // parser.
     int result = isAbstract ? abstractMask : 0;
     if (modifiers == null) return result;
     for (Modifier modifier in modifiers) {
       result |= modifier.mask;
     }
     return result;
   }
 }