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

tests/language/null_bottom_test.dart

Index: tests/language/null_bottom_test.dart
diff --git a/tests/language/null_bottom_test.dart b/tests/language/null_bottom_test.dart
index f1288b741f29f35ea5a70d8720971c027168eadb..fb14e9f0de9573e85a1393e4fce11b5061be7bb5 100644
--- a/tests/language/null_bottom_test.dart
+++ b/tests/language/null_bottom_test.dart
@@ -7,11 +7,15 @@ import "package:expect/expect.dart";
 
 // Test that `Null` acts like the bottom type - less than any other type.
 
-bool isCheckedMode = () {
-  bool c = false;
-  assert(c = true);
-  return c;
-}();
+final bool isCheckedMode = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
 
 typedef R Fun<A, R>(A argument);