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

tests/corelib/splay_tree_from_iterable_test.dart

Index: tests/corelib/splay_tree_from_iterable_test.dart
diff --git a/tests/corelib/splay_tree_from_iterable_test.dart b/tests/corelib/splay_tree_from_iterable_test.dart
index c60293a1eed5a7bd9a33d3d124b0eec40cc59b92..70f30baa1e7f24104c7ecd8554f72290ffa9b8fb 100644
--- a/tests/corelib/splay_tree_from_iterable_test.dart
+++ b/tests/corelib/splay_tree_from_iterable_test.dart
@@ -119,10 +119,18 @@ void genericTypeTest() {
   Expect.isFalse(map is SplayTreeMap<dynamic, int>);
 }
 
+final bool isCheckedMode = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
 // Test in checked mode with explicitly given types.
 void typedTest() {
-  bool isCheckedMode = false;
-  assert((isCheckedMode = true));
   if (!isCheckedMode) return;
 
   // Assign functions to untyped function variables.