Handle switch statement in simple types inferrer.

tests/language/issue10581_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 https://code.google.com/p/dart/issues/detail?id=10581.
+
+import 'package:expect/expect.dart';
+
+abstract class AxesObject {
+  Update();
+}
+
+String result = '';
+
+class Point2DObject extends AxesObject {
+  Update() { result += 'P'; }
+}
+
+class BestFitObject extends AxesObject {
+  Update() { result += 'B'; }
+}
+
+class Foo {
+  AddAxesObject(type) {
+    AxesObject a = null;
+    switch (type) {
+      case 100:
+        a = new Point2DObject();
+        break;
+      case 200:
+        a = new BestFitObject();
+        break;
+    }
+    if (a != null) {
+      a.Update();
+    }
+  }
+  AddAxesObject2(type) {
+    AxesObject a = null;
+    if (type == 100) {
+      a = new Point2DObject();
+    } else if (type == 200) {
+      a = new BestFitObject();
+    }
+    if (a != null) {
+      a.Update();
+    }
+  }
+}
+
+main() {
+  var f = new Foo();
+  f.AddAxesObject(100);
+  f.AddAxesObject(200);
+  f.AddAxesObject2(100);
+  f.AddAxesObject2(200);
+  Expect.equals('PBPB', result);
+}