fix missing digit error translation

pkg/front_end/lib/src/scanner/errors.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.
 
 import 'package:front_end/src/base/errors.dart';
 import 'package:front_end/src/fasta/scanner/error_token.dart';
 import 'package:front_end/src/fasta/scanner/token.dart';
 import 'package:front_end/src/fasta/scanner/token_constants.dart';
 
 /**
@@ skipping 87 matching lines @@
     case ErrorKind.UnmatchedToken:
       return null;
     case ErrorKind.UnterminatedComment:
       // TODO(paulberry,ahe): Fasta reports the error location as the entire
       // comment; analyzer expects the end of the comment.
       charOffset = endOffset;
       return _makeError(ScannerErrorCode.UNTERMINATED_MULTI_LINE_COMMENT, null);
     case ErrorKind.MissingExponent:
       // TODO(paulberry,ahe): Fasta reports the error location as the entire
       // number; analyzer expects the end of the number.
-      charOffset = endOffset;
+      charOffset = endOffset - 1;
       return _makeError(ScannerErrorCode.MISSING_DIGIT, null);
     case ErrorKind.ExpectedHexDigit:
       // TODO(paulberry,ahe): Fasta reports the error location as the entire
       // number; analyzer expects the end of the number.
-      charOffset = endOffset;
+      charOffset = endOffset - 1;
       return _makeError(ScannerErrorCode.MISSING_HEX_DIGIT, null);
     case ErrorKind.NonAsciiIdentifier:
     case ErrorKind.NonAsciiWhitespace:
       return _makeError(ScannerErrorCode.ILLEGAL_CHARACTER, [token.character]);
     case ErrorKind.UnexpectedDollarInString:
       return null;
     default:
       throw new UnimplementedError('$errorCode');
   }
 }
 
 /**
  * Determines whether the given [charOffset], which came from the non-EOF token
  * [token], represents the end of the input.
  */
 bool _isAtEnd(Token token, int charOffset) {
   while (true) {
     // Skip to the next token.
     token = token.next;
     // If we've found an EOF token, its charOffset indicates where the end of
     // the input is.
     if (token.isEof) return token.charOffset == charOffset;
     // If we've found a non-error token, then we know there is additional input
     // text after [charOffset].
     if (token.info.kind != BAD_INPUT_TOKEN) return false;
     // Otherwise keep looking.
   }
 }