Introduce DartType super hierarchy based on Entity

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 41 matching lines @@
   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<Null>.generate(5).elementAt(2);

[Siggi Cherem (dart-lang), 2016/12/22 15:42:03]

not sure I follow: why is this change needed?

[Johnni Winther, 2016/12/22 15:58:49]

From a different CL. Removed.

+  });
   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);
 }