Disallow double in case clauses and const map literal keys

tests/language/switch_bad_case_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.
+
+// Test reporting a compile-time error if case expressions do not all have
+// the same type or are of type double.
+
+import "package:expect/expect.dart";
+
+void main() {
+  Expect.equals("IV", caesarSays(4));
+  Expect.equals(null, caesarSays(2));
+  Expect.equals(null, archimedesSays(3.14));
+}
+
+caesarSays(n) {
+  switch (n) {
+    case 1:
+      return "I";
+    case 4:
+      return "IV";
+    case "M":         /// 01: compile-time error
+      return 1000;    /// 01: continued
+  }
+  return null;
+}
+
+archimedesSays(n) {
+  switch (n) {        /// 02: continued
+    case 3.14:        /// 02: compile-time error
+      return "Pi";    /// 02: continued
+    case 2.71828:     /// 02: continued
+      return "Huh?";  /// 02: continued
+  }                   /// 02: continued
+  return null;
+}