Fix all strong mode warnings.

lib/src/parser.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:source_span/source_span.dart';
 
 import 'ast.dart';
 import 'scanner.dart';
 import 'token.dart';
 
@@ skipping 78 matching lines @@
 
       case TokenType.leftParen:
         var child = _conditional();
         if (!_scanner.scan(TokenType.rightParen)) {
           throw new SourceSpanFormatException(
               'Expected ")".', _scanner.peek().span);
         }
         return child;
 
       case TokenType.identifier:
-        return new VariableNode(token.name, token.span);
+        return new VariableNode((token as IdentifierToken).name, token.span);
 
       default:
         throw new SourceSpanFormatException("Expected expression.", token.span);
     }
   }
 }