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

tests/corelib/splay_tree_from_iterable_test.dart

 // Copyright (c) 2011, 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.
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 
 main() {
   defaultFunctionValuesTest();
   defaultKeyFunctionTest();
@@ skipping 103 matching lines @@
   Expect.isTrue(map is Map<int, String>);
   Expect.isTrue(map is SplayTreeMap<int, String>);
 
   // Make sure it is not just SplayTreeMap<dynamic, dynamic>.
   Expect.isFalse(map is SplayTreeMap<String, dynamic>);
   Expect.isFalse(map is SplayTreeMap<dynamic, int>);
 }
 
 // Test in checked mode with explicitly given types.
 void typedTest() {
-  bool isCheckedMode = false;
-  assert((isCheckedMode = true));
-  if (!isCheckedMode) return;
+  if (!typeAssertionsEnabled) return;
 
   // Assign functions to untyped function variables.
   Function key = (int v) => "$v";
   Function value = (int v) => v.isOdd;
   Function id = (int i) => i;
 
   Expect.throws(() {
     new SplayTreeMap<String, bool>.fromIterable(<int>[1, 2, 3], key: key
         // No "value" map, defaults to identity, which returns int, not bool.
         );
@@ skipping 23 matching lines @@
       key: key, value: value);
   Iterable<String> keys = map.keys;
   Iterable<bool> values = map.values;
   List<String> keyList = keys.toList();
   List<bool> valueList = values.toList();
   Expect.equals(3, keyList.length);
   Expect.equals(3, valueList.length);
   Expect.equals(keys.first, map.firstKey());
   Expect.equals(keys.last, map.lastKey());
 }