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

tests/language/function_type/function_type30_test.dart

Index: tests/language/function_type/function_type30_test.dart
diff --git a/tests/language/function_type/function_type30_test.dart b/tests/language/function_type/function_type30_test.dart
index 03fe368bda472214896b1b7d42a7712d422d1af2..ede058e07857d07ee1281721f0fbb52aff8fc873 100644
--- a/tests/language/function_type/function_type30_test.dart
+++ b/tests/language/function_type/function_type30_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({List<Function> x});
 typedef F1<T> = List<Function> Function(int x, [Function x2]);