generate synthetic and error tokens when missing digit

pkg/front_end/test/scanner_fasta_test.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.
 import 'dart:convert';
 
 import 'package:analyzer/src/fasta/token_utils.dart';
 import 'package:front_end/src/fasta/fasta_codes.dart';
 import 'package:front_end/src/fasta/scanner.dart' as usedForFuzzTesting;
 import 'package:front_end/src/fasta/scanner/error_token.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/string_scanner.dart' as fasta;
@@ skipping 207 matching lines @@
     // Remove the last token.
     {
       prepareTokens();
       c3.remove();
       expect(token.precedingComments, c1);
       expect(c1.next, c2);
       expect(c2.next, isNull);
     }
   }
 
+  void test_double_error() {
+    String source = "3457e";
+    ErrorListener listener = new ErrorListener();
+    Token token = scanWithListener(source, listener);
+    expect(token, isNotNull);
+    expect(token.type, TokenType.DOUBLE);
+    expect(token.offset, 0);
+    expect(token.isSynthetic, isTrue);
+    // the invalid token is updated to be valid ...
+    expect(token.lexeme, source + "0");
+    // ... but the length does *not* include the additional character
+    // so as to be true to the original source.
+    expect(token.length, source.length);
+    expect(token.next.isEof, isTrue);
+    expect(listener.errors, hasLength(1));
+    TestError error = listener.errors[0];
+    expect(error.errorCode, ScannerErrorCode.MISSING_DIGIT);
+    expect(error.offset, source.length - 1);
+  }
+
   @override
   void test_mismatched_opener_in_interpolation() {
     // When openers and closers are mismatched,
     // fasta favors considering the opener to be mismatched
     // and inserts synthetic closers as needed.
     // r'"${({(}}"' is parsed as r'"${({()})}"'
     // where both ')' are synthetic
     ErrorListener listener = new ErrorListener();
     BeginToken interpolationStart =
         scanWithListener(r'"${({(}}"', listener).next;
@@ skipping 508 matching lines @@
   final ErrorListener _listener;
 
   ToAnalyzerTokenStreamConverter_WithListener(this._listener);
 
   @override
   void reportError(
       ScannerErrorCode errorCode, int offset, List<Object> arguments) {
     _listener.errors.add(new TestError(offset, errorCode, arguments));
   }
 }