Change assert implementation to not depend on a top-level function called 'assert'.

tests/compiler/dart2js/unparser_test.dart

Index: tests/compiler/dart2js/unparser_test.dart
diff --git a/tests/compiler/dart2js/unparser_test.dart b/tests/compiler/dart2js/unparser_test.dart
index 5186a136c9d0d4a83ea5357942289b8446eecd7f..fc69676cd3811fc5f7c8b2b59431b3ae5ff4a338 100644
--- a/tests/compiler/dart2js/unparser_test.dart
+++ b/tests/compiler/dart2js/unparser_test.dart
@@ -35,7 +35,7 @@ interface Map {}
 interface Closure {}
 interface Dynamic {}
 interface Null {}
-assert() {}
+_assert() {}
 class Math {
   static double parseDouble(String s) => 1.0;
 }