Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/parser/parser.dart

 // Copyright (c) 2012, 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.parser.parser;
 
 import '../scanner.dart' show
     ErrorToken;
 
 import '../scanner/recover.dart' show
@@ skipping 2437 matching lines @@
       mayParseFunctionExpressions = true;
       token = parseParenthesizedExpression(token);
       mayParseFunctionExpressions = old;
       return token;
     }
   }
 
   Token parseParenthesizedExpression(Token token) {
     // We expect [begin] to be of type [BeginGroupToken], but we don't know for
     // sure until after calling expect.
-    var begin = token;
+    BeginGroupToken begin = token;

[Siggi Cherem (dart-lang), 2017/02/02 04:26:58]

strong mode infers `var` to be the type of the rhs, which is Token.

[ahe, 2017/02/02 07:28:30]

The type should be dynamic. This cast isn't correct. Take a look at the comment
above.

[kasperl, 2017/02/02 07:51:30]

var first = token;
token = expect('(', token);
BeginGroupToken begin = first;

seems cleaner to me. 

[Siggi Cherem (dart-lang), 2017/02/06 23:35:50]

Sorry - I totally missed the comment. Thanks for clarifying. I went ahead with
Kasper's suggestion here.

     token = expect('(', token);
     // [begin] is now known to have type [BeginGroupToken].
     token = parseExpression(token);
     if (!identical(begin.endGroup, token)) {
       reportUnrecoverableError(token, ErrorKind.UnexpectedToken);
       token = begin.endGroup;
     }
     listener.handleParenthesizedExpression(begin);
     return expect(')', token);
   }
@@ skipping 847 matching lines @@
         break;
     }
     if (isRecoverable) {
       listener.handleRecoverableError(token, kind, arguments);
       return null;
     } else {
       return listener.handleUnrecoverableError(token, kind, arguments);
     }
   }
 }