remove old fasta missing exponent recovery

pkg/front_end/test/scanner_replacement_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' show UTF8;
 
 import 'package:front_end/src/fasta/scanner/recover.dart'
     show defaultRecoveryStrategy;
 import 'package:front_end/src/fasta/scanner.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/token.dart' as fasta;
@@ skipping 72 matching lines @@
     fasta.Token close = open.next;
     expect(close.next.isEof, isTrue);
     expect(open.endGroup, close);
     expect(open.isSynthetic, isFalse);
     expect(close.isSynthetic, isTrue);
     listener.assertErrors([
       new TestError(0, ScannerErrorCode.EXPECTED_TOKEN, [expectedCloser]),
     ]);
   }
 
+  void test_double_error() {
+    String source = "3457e";
+    ErrorListener listener = new ErrorListener();
+    analyzer.Token token = scanWithListener(source, listener);
+    expect(token.type, analyzer.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);
+  }
+
   void test_lt() {
     // fasta does not automatically insert a closer for '<'
     // because it could be part of an expression rather than an opener
     analyzer.BeginToken lt = _scan('<');
     expect(lt.next.isEof, isTrue);
     expect(lt.isSynthetic, isFalse);
   }
 
   void test_lt_gt() {
     _assertOpenClosePair('< >');
@@ skipping 157 matching lines @@
       } else if (token is fasta.UnmatchedToken) {
         expect(lastClosedGroup?.endGroup?.next, same(token),
             reason: 'Unexpected error token for group: $lastClosedGroup');
         expect(token.begin, lastClosedGroup);
       }
       token = token.next;
     }
     expect(openerStack, isEmpty, reason: 'Missing closers');
   }
 }