Make VM accept function values in assert statements.

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 98814f8dff413ca96567b8366a1d8f46d06133ee..8c995a4d146537d169781e1a088bdb710545f0e2 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -26,6 +26,11 @@ class _AssertionError extends Error implements AssertionError {
   static _throwNew(int assertionStart, int assertionEnd)
       native "AssertionError_throwNew";
 
+  static _handleCondition(condition) {

[hausner, 2016/06/14 17:51:31]

I'd call this function _evaluateCondition(), or something else appropriate if we
combine evaluation and the throw into one.

+    if (condition is Function) return condition();
+    return condition;
+  }
+
   String toString() {
     if (_url == null) {
       return _failedAssertion;