Make generator argument to Iterable.generate optional.

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";
+
+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) => "$x"));
+  test([2,3,4,5,6], new Iterable.generate(7).skip(2));
+  test([0,1,2,3,4], new Iterable.generate(7).take(5));
+  test([], new Iterable.generate(5).skip(6));
+  test([], new Iterable.generate(5).take(0));
+  test([], new Iterable.generate(5).take(3).skip(3));
+  test([], new Iterable.generate(5).skip(6).take(0));
+
+  // Test types.
+
+  Iterable<int> it = new Iterable<int>.generate(5);
+  Expect.isTrue(it is Iterable<int>);
+  Expect.isTrue(it.iterator is Iterator<int>);
+  Expect.isTrue(it is! Iterable<String>);
+  Expect.isTrue(it.iterator is! Iterator<String>);
+  test([0,1,2,3,4], it);
+
+  Iterable<String> st = new Iterable<String>.generate(5, (x) => "$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) {
+    Expect.throws(() => new Iterable<String>.generate(5));
+  }
+}