Move validation to validate.dart and add comments.

tests/language/critical_edge2_test.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.
 
 // This test broke dart2js.
 // A compiler must not construct a critical edge on this program.
+//
+// See critical_edge_test.dart for a description of the problem.
 
 import "package:expect/expect.dart";
 
 String parse(String uri) {
   int index = 0;
   int char = -1;
 
   void parseAuth() {
     index;
     char;
@@ skipping 10 matching lines @@
     index++;
   }
 
   print(char);
   return "bad";
 }
 
 main() {
   Expect.equals("good", parse("dart:_foreign_helper"));
 }