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

tests/corelib/int_parse_radix_test.dart

Index: tests/corelib/int_parse_radix_test.dart
diff --git a/tests/corelib/int_parse_radix_test.dart b/tests/corelib/int_parse_radix_test.dart
index 9473631bebbf955ebdb998988cd04fd6ac07e1b6..643d737c67b865085835d1e9e311cde33dfea4e1 100644
--- a/tests/corelib/int_parse_radix_test.dart
+++ b/tests/corelib/int_parse_radix_test.dart
@@ -5,9 +5,17 @@
 import "package:expect/expect.dart";
 import "dart:math" show pow;
 
+final bool isCheckedMode = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
 void main() {
-  bool checkedMode = false;
-  assert((checkedMode = true));
   const String oneByteWhiteSpace = "\x09\x0a\x0b\x0c\x0d\x20"
     "\x85" //   //# 01: ok
       "\xa0";
@@ -116,7 +124,7 @@ void main() {
   }
 
   testBadTypes(var source, var radix) {
-    if (!checkedMode) {
+    if (!isCheckedMode) {
       // 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));