Add some more validation of things in expect that should not be used.

tools/migration/lib/src/validate.dart

 // Copyright (c) 2017, 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 'log.dart';
 
 final _unittestPattern = "package:unittest";
 final _checkedPattern = new RegExp(r"\bchecked\b");
 final _abstractErrorPattern =
     new RegExp(r"\bAbstractClassInstantiationError\b");
 final _typeErrorPattern = new RegExp(r"\bTypeError\b");
+final _typeAssertionsEnabledPattern = new RegExp(r"\btypeAssertionsEnabled\b");
+final _checkedModeEnabledPattern = new RegExp(r"\bcheckedModeEnabled\b");
 
 void validateFile(String path, String source, [List<String> todos]) {
   check(Pattern pattern, String noteMessage, String todo) {
     if (!source.contains(pattern)) return;
     note("${bold(path)} $noteMessage.");
     if (todos != null) todos.add(todo);
   }
 
   check(_unittestPattern, "uses the unittest package", "Migrate off unittest.");
   check(_checkedPattern, 'mentions "checked"',
       'Fix code that mentions "checked" mode.');
   check(_abstractErrorPattern, 'mentions "AbstractClassInstantiationError"',
       "Remove code that checks for AbstractClassInstantiationError.");
   check(_typeErrorPattern, 'mentions "TypeError"',
       "Ensure code that checks for a TypeError uses 2.0 semantics.");
+  check(_typeAssertionsEnabledPattern, 'mentions "typeAssertionsEnabled"',
+      "Remove checks for typeAssertionsEnabled, they are always enabled in 2.0.");
+  check(_checkedModeEnabledPattern, 'mentions "typeAssertionsEnabled"',
+      "Remove checks for checkedModeEnabled, it is always enabled in 2.0.");
 }