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

tests/corelib/int_parse_radix_test.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.
 
 import "package:expect/expect.dart";
 import "dart:math" show pow;
 
 void main() {
-  bool checkedMode = false;
-  assert((checkedMode = true));
   const String oneByteWhiteSpace = "\x09\x0a\x0b\x0c\x0d\x20"
     "\x85" //   //# 01: ok
       "\xa0";
   const String whiteSpace = "$oneByteWhiteSpace\u1680"
       "\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a"
       "\u2028\u2029\u202f\u205f\u3000\ufeff";
 
   var digits = "0123456789abcdefghijklmnopqrstuvwxyz";
   var zeros = "0" * 64;
 
@@ skipping 88 matching lines @@
   testFails("+ 1", 2); // No space between sign and digits.
   testFails("- 1", 2); // No space between sign and digits.
   testFails("0x", null);
   for (int i = 2; i <= 33; i++) {
     // No 0x specially allowed.
     // At radix 34 and above, "x" is a valid digit.
     testFails("0x10", i);
   }
 
   testBadTypes(var source, var radix) {
-    if (!checkedMode) {
+    if (!typeAssertionsEnabled) {
       // No promises on what error is thrown if the type doesn't match.
       // Likely either ArgumentError or NoSuchMethodError.
       Expect.throws(() => int.parse(source, radix: radix, onError: (s) => 0));
       return;
     }
-    // In checked mode, it's always a TypeError.
+    // With type assertions enabled we can be more precise.
     Expect.throws(() => int.parse(source, radix: radix, onError: (s) => 0),
         (e) => e is TypeError || e is CastError);
   }
 
   testBadTypes(9, 10);
   testBadTypes(true, 10);
   testBadTypes("0", true);
   testBadTypes("0", "10");
 
   testBadArguments(String source, int radix) {
     // If the types match, it should be an ArgumentError of some sort.
     Expect.throws(() => int.parse(source, radix: radix, onError: (s) => 0),
         (e) => e is ArgumentError);
   }
 
   testBadArguments("0", -1);
   testBadArguments("0", 0);
   testBadArguments("0", 1);
   testBadArguments("0", 37);
 
   // See also int_parse_radix_bad_handler_test.dart
 }
 
 bool isFail(e) => e == "FAIL";