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

tests/corelib/iterable_generate_test.dart

 // Copyright (c) 2013, 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";
 
+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());
   }
 
   test([], new Iterable.generate(0));
   test([0], new Iterable.generate(1));
   test([0, 1, 2, 3, 4], new Iterable.generate(5));
   test(["0", "1", "2", "3", "4"], new Iterable.generate(5, (x) => "$x"));
   test([2, 3, 4, 5, 6], new Iterable.generate(7).skip(2));
@@ skipping 12 matching lines @@
   Expect.isTrue(it.iterator is! Iterator<String>);
   test([0, 1, 2, 3, 4], it);
 
   Iterable<String> st = new Iterable<String>.generate(5, (x) => "$x");
   Expect.isTrue(st is Iterable<String>);
   Expect.isTrue(st.iterator is Iterator<String>);
   Expect.isFalse(st is Iterable<int>);
   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));
   }
 
   // Omitted generator function means `(int x) => x`, and the type parameters
   // must then be compatible with `int`.
   // Check that we catch invalid type parameters.
 
   // Valid types:
   Iterable<int> iter1 = new Iterable<int>.generate(5);
   Expect.equals(2, iter1.elementAt(2));
   Iterable<num> iter2 = new Iterable<num>.generate(5);
   Expect.equals(2, iter2.elementAt(2));
   Iterable<Object> iter3 = new Iterable<Object>.generate(5);
   Expect.equals(2, iter3.elementAt(2));
   Iterable<dynamic> iter4 = new Iterable<dynamic>.generate(5);
   Expect.equals(2, iter4.elementAt(2));
 
   // 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
   } //                                                                  //# 01: continued
   Expect.throws(() => new Iterable<bool>.generate(5));
 
   // Regression: https://github.com/dart-lang/sdk/issues/26358
   var count = 0;
   var iter = new Iterable.generate(5, (v) {
     count++;
     return v;
   });
   Expect.equals(0, count);
   Expect.equals(2, iter.elementAt(2)); // Doesn't compute the earlier values.
   Expect.equals(1, count);
   Expect.equals(2, iter.skip(2).first); // Doesn't compute the skipped values.
   Expect.equals(2, count);
 }