Improve type information and minor clean up

pkg/analyzer/lib/src/dart/ast/utilities.dart

 // Copyright (c) 2014, 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 analyzer.src.dart.ast.utilities;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 2299 matching lines @@
     return list;
   }
 
   @override
   Object visitMapLiteral(MapLiteral node) {
     HashMap<String, Object> map = new HashMap<String, Object>();
     for (MapLiteralEntry entry in node.entries) {
       Object key = entry.key.accept(this);
       Object value = entry.value.accept(this);
       if (key is String && !identical(value, NOT_A_CONSTANT)) {
-        map[(key as String)] = value;
+        map[key] = value;
       } else {
         return NOT_A_CONSTANT;
       }
     }
     return map;
   }
 
   @override
   Object visitMethodInvocation(MethodInvocation node) => visitNode(node);
 
@@ skipping 5455 matching lines @@
    * Safely visit the given [token], printing the [suffix] after the token if it
    * is non-`null`.
    */
   void _visitTokenWithSuffix(Token token, String suffix) {
     if (token != null) {
       _writer.print(token.lexeme);
       _writer.print(suffix);
     }
   }
 }