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;
-}();
-
 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 24 matching lines @@
   Expect.isNotNull(ccn as C<C<Null>>);
   Expect.isNotNull(ccn as C<C<Object>>);
   Expect.isNotNull(ccn as C<C<int>>);
 
   var ci = new C<int>();
   Expect.isFalse(ci is C<Null>);
 
   var co = new C<Object>();
   Expect.isFalse(co is C<Null>);
 
-  if (!isCheckedMode) return;
+  if (!typeAssertionsEnabled) return;
 
   List<int> li1 = const <Null>[];
 
   C<Null> x1 = cn;
   C<Object> x2 = cn;
 
   Expect.identical(x1, cn);
   Expect.identical(x2, cn);
 
   const C<Null> cocn = const C<Null>();
@@ skipping 61 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);
 }