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

pkg/expect/lib/expect.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This library contains an Expect class with static methods that can be used
  * for simple unit-tests.
  */
 library expect;
 
@@ skipping 485 matching lines @@
   const TrustTypeAnnotations();
 }
 
 /// Annotation class for testing of dart2js. Use this as metadata on method
 /// declarations to disable closed world assumptions on parameters, effectively
 /// assuming that the runtime arguments could be any value. Note that the
 /// constraints due to [TrustTypeAnnotations] still apply.
 class AssumeDynamic {
   const AssumeDynamic();
 }
+
+/// Is true iff type assertions are enabled.
+final bool typeAssertionsEnabled = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
+/// Is true iff `assert` statements are enabled.
+final bool assertStatementsEnabled = (() {
+  try {
+    assert(false);
+  } on AssertionError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
+/// Is true iff checked mode is enabled.
+final bool checkedModeEnabled =
+    typeAssertionsEnabled && assertStatementsEnabled;