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

tests/language/null_bottom_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Dart test for testing the ternary operator.
 
 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);
 
 class C<T> {
   const C();
   T returns() => null;
   void accepts(T x) {}
 }
 
 class NullBound<T extends num> {}
@@ skipping 107 matching lines @@
   Null foo(Null x) => null;
   Null bar(Null x) => null;
   int baz(int x) => x;
   int qux(int x) => x;
 }
 
 // Avoid "variable not used" warnings.
 use(x) {
   return identical(x, x);
 }