Fix issue 12023 by avoiding a critical edge in the SSA graph with a switch statement.

tests/language/issue12023_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.
+
+import "package:expect/expect.dart";
+
+void main() {
+  List test = ["f", "5", "s", "6"];
+  int length = test.length;
+  for (int i = 0; i < length;) {
+    var action = test[i++];
+    switch (action) {
+      case "f":
+      case "s":
+        action = test[i - 1];
+        int value = int.parse(test[i++]);
+        if (action == "f") {
+          Expect.equals(5, value);
+        } else {
+          Expect.equals(6, value);
+        }
+        break;
+    }
+  }
+}