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

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
diff --git a/runtime/lib/errors_patch.dart b/runtime/lib/errors_patch.dart
index 69ae2195426a26a25a71c4346cb0d61cdf0e77a3..412f534ce7c13d0f71dd9a3306d79e20c04f9607 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -33,6 +33,9 @@ class _AssertionError extends Error implements AssertionError {
     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();
     }