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

tests/corelib/list_removeat_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';
 
 class MyList extends ListBase {
   List list;
   MyList(this.list);
 
   get length => list.length;
   set length(val) {
     list.length = val;
   }
 
   operator [](index) => list[index];
   operator []=(index, val) => list[index] = val;
 
   String toString() => "[" + join(", ") + "]";
 }
 
 // l1 must be a modifiable list with 5 elements from 0 to 4.
 void testModifiableList(l1) {
-  bool checkedMode = false;
-  assert(checkedMode = true);
-
   // Index must be integer and in range.
   Expect.throws(() {
     l1.removeAt(-1);
   }, (e) => e is RangeError, "negative");
   Expect.throws(() {
     l1.removeAt(5);
   }, (e) => e is RangeError, "too large");
   Expect.throws(() {
     l1.removeAt(null);
   }, (e) => e is ArgumentError, "too large");
   Expect.throws(() {
     l1.removeAt("1");
-  }, (e) => (checkedMode ? e is TypeError : e is ArgumentError), "string");
+  }, (e) => (typeAssertionsEnabled ? e is TypeError : e is ArgumentError),
+      "string");
   Expect.throws(() {
     l1.removeAt(1.5);
-  }, (e) => (checkedMode ? e is TypeError : e is ArgumentError), "double");
+  }, (e) => (typeAssertionsEnabled ? e is TypeError : e is ArgumentError),
+      "double");
 
   Expect.equals(2, l1.removeAt(2), "l1-remove2");
   Expect.equals(1, l1[1], "l1-1[1]");
 
   Expect.equals(3, l1[2], "l1-1[2]");
   Expect.equals(4, l1[3], "l1-1[3]");
   Expect.equals(4, l1.length, "length-1");
 
   Expect.equals(0, l1.removeAt(0), "l1-remove0");
   Expect.equals(1, l1[0], "l1-2[0]");
@@ skipping 19 matching lines @@
   Expect.throws(() {
     l3.removeAt(2);
   }, (e) => e is UnsupportedError, "unmodifiable");
 
   // Empty list is not special.
   var l4 = [];
   Expect.throws(() {
     l4.removeAt(0);
   }, (e) => e is RangeError, "empty");
 }