Add support to dart2js for option --enable-asserts.

tests/language/function_type/function_type20_test.dart

Index: tests/language/function_type/function_type20_test.dart
diff --git a/tests/language/function_type/function_type20_test.dart b/tests/language/function_type/function_type20_test.dart
index e6540045ee3a678ae320ef335976bf8b55a0f440..5738e876007140e8fc9be0395bab65fadac31be6 100644
--- a/tests/language/function_type/function_type20_test.dart
+++ b/tests/language/function_type/function_type20_test.dart
@@ -19,9 +19,15 @@ import 'package:expect/expect.dart';
 @AssumeDynamic()
 confuse(f) => f;
 
-final bool inCheckedMode =
-    (() { bool result = false; assert(result = true); return result; })();
-
+final bool inCheckedMode = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
 
 typedef F0<T> = int Function(int x0, {Function x});
 typedef F1<T> = List<Function> Function({int x});