fix PathObserver to avoid try+catch

pkg/observe/lib/transform.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Code transform for @observable. The core transformation is relatively
  * straightforward, and essentially like an editor refactoring.
  */
 library observe.transform;
 
@@ skipping 238 matching lines @@
     var pos = cls.extendsClause.end;
     code.edit(pos, pos, ' with ChangeNotifier ');
   } else {
     var params = cls.typeParameters;
     var pos =  params != null ? params.end : cls.name.end;
     code.edit(pos, pos, ' extends ChangeNotifier ');
   }
 }
 
 SimpleIdentifier _getSimpleIdentifier(Identifier id) =>
-    id is PrefixedIdentifier ? (id as PrefixedIdentifier).identifier : id;
+    id is PrefixedIdentifier ? id.identifier : id;
 
 
 bool _hasKeyword(Token token, Keyword keyword) =>
-    token is KeywordToken && (token as KeywordToken).keyword == keyword;
+    token is KeywordToken && token.keyword == keyword;
 
 String _getOriginalCode(TextEditTransaction code, ASTNode node) =>
     code.original.substring(node.offset, node.end);
 
 void _fixConstructor(ConstructorDeclaration ctor, TextEditTransaction code,
     Set<String> changedFields) {
 
   // Fix normal initializers
   for (var initializer in ctor.initializers) {
     if (initializer is ConstructorFieldInitializer) {
@@ skipping 132 matching lines @@
 
 Token _findFieldSeperator(Token token) {
   while (token != null) {
     if (token.type == TokenType.COMMA || token.type == TokenType.SEMICOLON) {
       break;
     }
     token = token.next;
   }
   return token;
 }