Skip the type test for expressions which have already evaluated to true or false.

runtime/lib/errors_patch.dart

 // Copyright (c) 2012, 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.
 
 @patch class Error {
   @patch static String _objectToString(Object object) {
     return Object._toString(object);
   }
 
   @patch static String _stringToSafeString(String string) {
@@ skipping 15 matching lines @@
   // out of the script. It expects a Dart stack frame from class
   // _AssertionError. Thus we need a Dart stub that calls the native code.
   static _throwNew(int assertionStart, int assertionEnd, Object message) {
     _doThrowNew(assertionStart, assertionEnd, message);
   }
 
   static _doThrowNew(int assertionStart, int assertionEnd, Object message)
     native "AssertionError_throwNew";
 
   static _evaluateAssertion(condition) {
+    if (identical(condition, true) || identical(condition, false)) {
+      return condition;
+    }
     if (condition is Function) {

[rmacnak, 2017/02/02 19:02:05]

It might be worth adding

if (condition is _Closure) {
  return condition();
}

[Cutch, 2017/02/02 19:57:49]

Done.

       condition = condition();
     }
     return condition;
   }
 
   String get _messageString {
     if (message == null) return "is not true.";
     if (message is String) return message;
     return Error.safeToString(message);
   }
@@ skipping 321 matching lines @@
     return msg_buf.toString();
   }
 }
 
 
 class _CompileTimeError extends Error {
   final String _errorMsg;
   _CompileTimeError(this._errorMsg);
   String toString() => _errorMsg;
 }