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

tests/language/function_type/function_type15_test.dart

Index: tests/language/function_type/function_type15_test.dart
diff --git a/tests/language/function_type/function_type15_test.dart b/tests/language/function_type/function_type15_test.dart
index e5733e1a9ff7e9b1bea95680b5d7a369c356c537..a265f636303e368b5dd7b4a664af1a154f4ef367 100644
--- a/tests/language/function_type/function_type15_test.dart
+++ b/tests/language/function_type/function_type15_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(Function x0);
 typedef F1<T> = List<Function> Function(int y, [int x]);