Make EfficientLength extend Iterable.

tests/language/efficient_length_warning_test.dart

+// Copyright (c) 2016, 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.
+// Third dart test program.
+
+import "dart:math";
+
+main() {
+  // This should no longer cause a warning because the least-upper-bound
+  // of List<int> and Set<int> is Object.
+  // The LUB is now EfficientLengthIterable and it extends Iterable.
+  var x = (new Random().nextBool() // Unpredictable condition.
+      ? <int>[1]
+      : new Set<int>.from([1])).first;
+  if (x != 1) throw "Wat?";
+}
+