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

tests/corelib/iterable_generate_test.dart

Index: tests/corelib/iterable_generate_test.dart
diff --git a/tests/corelib/iterable_generate_test.dart b/tests/corelib/iterable_generate_test.dart
index 958f56063d8cb6de3fef17cbbbbecead6d379318..8302e1f7bf2d9c365551be99109399cce4db34f3 100644
--- a/tests/corelib/iterable_generate_test.dart
+++ b/tests/corelib/iterable_generate_test.dart
@@ -4,9 +4,17 @@
 
 import "package:expect/expect.dart";
 
+final bool isCheckedMode = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
 main() {
-  bool checkedMode = false;
-  assert((checkedMode = true));
   void test(expectedList, generatedIterable) {
     Expect.equals(expectedList.length, generatedIterable.length);
     Expect.listEquals(expectedList, generatedIterable.toList());
@@ -39,7 +47,7 @@ main() {
   Expect.isFalse(st.iterator is Iterator<int>);
   test(["0", "1", "2", "3", "4"], st);
 
-  if (checkedMode) {
+  if (isCheckedMode) {
     Expect.throws(() => new Iterable<String>.generate(5));
   }
 
@@ -59,8 +67,8 @@ main() {
 
   // Invalid types:
   Expect.throws(() => new Iterable<String>.generate(5));
-  if (checkedMode) { //                                                 //# 01: ok
-    Expect.throws(() => new Iterable<Null>.generate(5).elementAt(2)); //# 01: continued
+  if (isCheckedMode) { //                                               //# 01: ok
+    Expect.throws(() => new Iterable<Null>.generate(5).elementAt(2));   //# 01: continued
   } else { //                                                           //# 01: continued
     Iterable<dynamic> iter5 = new Iterable<Null>.generate(5); //        //# 01: continued
     Expect.equals(2, iter5.elementAt(2)); //                            //# 01: continued