Improve position information.

pkg/front_end/lib/src/fasta/quote.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.quote;
 
 import 'errors.dart' show inputError, internalError;
 
 import 'scanner/characters.dart'
     show
@@ skipping 142 matching lines @@
     case Quote.RawSingle:
     case Quote.RawDouble:
       return string;
 
     case Quote.RawMultiLineSingle:
     case Quote.RawMultiLineDouble:
       return !string.contains("\r")
           ? string
           : unescapeCodeUnits(string.codeUnits, true);
   }
-  return internalError("Unhandled string quote: $quote");
+  return internalError("Internal error: Unexpected quote: $quote.");
 }
 
 const String incompleteSequence = "Incomplete escape sequence.";
 
 const String invalidCharacter = "Invalid character in escape sequence.";
 
 const String invalidCodePoint = "Invalid code point.";
 
 // Note: based on
 // [StringValidator.validateString](pkg/compiler/lib/src/string_validator.dart).
@@ skipping 71 matching lines @@
         }
       } else {
         // Nothing, escaped character is passed through;
       }
       if (code > 0x10FFFF) return error(i, invalidCodePoint);
     }
     result[resultOffset++] = code;
   }
   return new String.fromCharCodes(result, 0, resultOffset);
 }