Implement the new definition of flatten() from r44569 in analyzer.

tests/language/flatten_test.dart

+// Copyright (c) 2015, 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 'dart:async';
+
+class Derived<T> implements Future<T> {
+  noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+class FixedPoint<T> implements Future<FixedPoint<T>> {
+  noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+class Divergent<T> implements Future<Divergent<Divergent<T>>> {
+  noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+
+test() async {
+  // flatten(Derived<int>) = int
+  int x = await new Derived<int>(); /// 01: runtime error
+  Future<int> f() async => new Derived<int>(); /// 02: ok
+  Future<int> f() async { return new Derived<int>(); } /// 03: ok
+  Future<int> x = (() async => new Derived<int>())(); /// 04: runtime error
+
+  // flatten(FixedPoint<int>) = FixedPoint<int>
+  FixedPoint<int> x = await new FixedPoint<int>(); /// 05: runtime error
+  Future<FixedPoint<int>> f() async => new FixedPoint<int>(); /// 06: ok
+  Future<FixedPoint<int>> f() async { return new FixedPoint<int>(); } /// 07: ok
+  Future<FixedPoint<int>> x = (() async => new FixedPoint<int>())(); /// 08: runtime error
+
+  // flatten(Divergent<int>) = Divergent<Divergent<int>>
+  Divergent<Divergent<int>> x = await new Divergent<int>(); /// 09: runtime error
+  Future<Divergent<Divergent<int>>> f() async => new Divergent<int>(); /// 10: ok
+  Future<Divergent<Divergent<int>>> f() async { return new Divergent<int>(); } /// 11: ok
+  Future<Divergent<Divergent<int>>> x = (() async => new Divergent<int>())(); /// 12: runtime error
+}
+
+main() {
+  test();
+}