Fix issue #27830: the type of an super-set assignment expression is the type of

tests/compiler/dart2js_extra/inference_super_set_call_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.
+
+// Regression test for dart2js: we incorrectly modeled `super.x = rhs` as a
+// call and not an assignment, so the type of the expression was incorrectly
+// assumed to be the return type of the setter rather than the type of the rhs.
+import 'package:expect/expect.dart';
+
+abstract class A {
+  set x(v) {}
+  set z(v) {}
+  set y(v) { return 'hi';}
+}
+
+class S extends A {
+  var _x;      // was bad: inferred as null, than [null | int]
+  var _y = ''; // was bad: inferred as String, rather than [String | int]
+  var _z;      // was ok : inferred as [null | int]
+
+  set x(v) {
+    _x = super.x = v;
+  }
+
+  set z(v) {
+    super.z = v;
+    _z = v;
+  }
+
+  set y(v) {
+    _y = super.y = v;
+  }
+
+  get isXNull => _x == null;
+  get isZNull => _z == null;
+}
+
+main() {
+  var s = new S()
+    ..x = 2
+    ..y = 2
+    ..z = 2;
+  Expect.equals(false, s.isXNull);      // was incorrectly optimized to 'true'
+  Expect.equals(false, s._y is String); // was incorrectly optimized to 'true'
+  Expect.equals(false, s.isZNull);      // prints false
+}