Make Iterable.generate use ListIterable as base implementation.

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));
@@ skipping 24 matching lines @@
   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));
   }
+
+  // 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));
+  Expect.throws(() => new Iterable<Null>.generate(5));
+  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);
 }