Change switch to give errors when cases don't follow the newest syntax.

lib/compiler/implementation/warnings.dart

Index: lib/compiler/implementation/warnings.dart
diff --git a/lib/compiler/implementation/warnings.dart b/lib/compiler/implementation/warnings.dart
index 322de1d4e8ec40cd2e324c30caca4f44abcd9ad9..fae9ed3304e375a5e8cdea1b6ad0482f5f6f8d48 100644
--- a/lib/compiler/implementation/warnings.dart
+++ b/lib/compiler/implementation/warnings.dart
@@ -130,11 +130,14 @@ class MessageKind {
       'super not allowed here');
   static const INVALID_CASE_DEFAULT = const MessageKind(
       'default only allowed on last case of a switch');
-  static const INVALID_CASE_EXPRESSION = const MessageKind(
-      'case expression is not a compile-time constant int or string.');
-  static const INVALID_CASE_EXPRESSION_TYPE = const MessageKind(
-      "case expressions don't all have the same type "
-      "(must be all int or String).");
+
+  static const SWITCH_CASE_TYPES_NOT_EQUAL = const MessageKind(
+      "case expressions don't all have the same type.");
+  static const SWITCH_CASE_VALUE_OVERRIDES_EQUALS = const MessageKind(
+      "case expression value overrides 'operator=='.");
+  static const SWITCH_INVALID = const MessageKind(
+      "switch cases contain invalid expressions.");
+
   static const INVALID_ARGUMENT_AFTER_NAMED = const MessageKind(
       'non-named argument after named argument');