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

tests/language/type_error_test.dart

Index: tests/language/type_error_test.dart
diff --git a/tests/language/type_error_test.dart b/tests/language/type_error_test.dart
index fc62c23b936709710aef4246c03d3eb91bf3e087..ab1cfab87103826e3e89ebb99ada5c802acc7776 100644
--- a/tests/language/type_error_test.dart
+++ b/tests/language/type_error_test.dart
@@ -132,10 +132,10 @@ checkTypeError(o) {
     print(o);
   } on TypeError catch (e) {
     print(e); // This might provoke an error.
-    if (assertionsEnabled) return; // Expected type error.
+    if (isCheckedMode) return; // Expected type error.
     rethrow; // Rethrow unexpected type error.
   }
-  if (assertionsEnabled) {
+  if (isCheckedMode) {
     throw 'expected TypeError';
   }
 }
@@ -162,6 +162,16 @@ checkCastError(o) {
   throw 'expected CastError';
 }
 
+final bool isCheckedMode = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
 bool assertionsEnabled = false;
 
 main() {