Add type inference for switch statements.

pkg/front_end/testcases/inference_new/switch.dart

+// Copyright (c) 2017, 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.
+
+/*@testedFeatures=inference*/
+library test;
+
+T f<T>() => null;
+
+class C<T> {
+  const C();
+}
+
+void test(C<int> x) {
+  switch (x) {
+    case const /*@typeArgs=int*/ C():
+      var /*@type=int*/ y = 0;
+      break;
+    default:
+      var /*@type=int*/ y = 0;
+      break;
+  }
+}
+
+main() {}