Only generate assert in checked mode.

lib/compiler/implementation/warnings.dart

 // Copyright (c) 2011, 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.
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static final GENERIC = const MessageKind('#{1}');
 
@@ skipping 158 matching lines @@
 
   static final ILLEGAL_SUPER_SEND = const MessageKind(
       "#{1} cannot be called on super");
 
   static final ADDITIONAL_TYPE_ARGUMENT = const MessageKind(
       "additional type argument");
 
   static final MISSING_TYPE_ARGUMENT = const MessageKind(
       "missing type argument");
 
+  static final MISSING_ARGUMENTS_TO_ASSERT = const MessageKind(
+      "missing arguments to assert");
+
   static final COMPILER_CRASHED = const MessageKind(
       "Error: The compiler crashed when compiling this element.");
 
   static final PLEASE_REPORT_THE_CRASH = const MessageKind('''
 The compiler is broken.
 
 When compiling the above element, the compiler crashed. It is not
 possible to tell if this is caused by a problem in your program or
 not. Regardless, the compiler should not crash.
 
@@ skipping 77 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, List<Type> arguments)
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, List<Type> arguments)
     : super(kind, arguments);
 }