Follow factory redirections when checking case expressions.

tests/language/switch_case_test.dart

+// Copyright (c) 2014, 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 "package:expect/expect.dart";
+
+class A {
+  const A();
+  const factory A.B() = B;
+  const factory A.C() = C;
+  const factory A.C2() = D;
+}
+
+class B implements A {
+  const B();
+
+  operator ==(o) => true;  /// 00: compile-time error
+}
+
+class C implements A {
+  final int x;
+  const C() : x = 0;
+  const C.fromD() : x = 1;
+}
+
+class D implements C {
+  const factory D() = C.fromD;
+}
+
+main() {
+  var o = new A();
+  switch (o) {
+    case const C(): Expect.fail("bad switch"); break;
+    case const A.C(): Expect.fail("bad switch"); break;
+    case const A.C2(): Expect.fail("bad switch"); break;
+    case const A(): Expect.fail("bad switch"); break;  /// 01: compile-time error
+  }
+
+  switch (o) {
+    case const A(): Expect.fail("bad switch"); break;
+    case const A.B(): Expect.fail("bad switch"); break;  /// 02: compile-time error
+  }
+
+  switch (o) {
+    case const B(): Expect.fail("bad switch"); break;   /// 00: continued
+    case const A.B(): Expect.fail("bad switch"); break;  /// 00: continued
+  }
+}