Complain about assignments to parenthesized expressions.

pkg/front_end/lib/src/fasta/parser/token_stream_rewriter.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 'package:front_end/src/fasta/errors.dart';
-import 'package:front_end/src/fasta/scanner/precedence.dart';
 import 'package:front_end/src/fasta/scanner/token.dart';
 
 /// Provides the capability of inserting tokens into a token stream by rewriting
 /// the previous token to point to the inserted token.
 ///
 /// This class has been designed to take advantage of "previousToken" pointers
 /// when they are present, but not to depend on them.  When they are not
 /// present, it uses heuristics to try to find the find the previous token as
 /// quickly as possible by walking through tokens starting at the start of the
 /// file.
@@ skipping 80 matching lines @@
         nextPos = pos.next;
         if (nextPos == null || nextPos.charOffset > target.charOffset) {
           return null;
         }
       }
       pos = nextPos;
     }
     return pos;
   }
 }