Add dart_messages package.

pkg/dart_messages/test/smoke_messages.dart

Index: pkg/dart_messages/test/smoke_messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/dart2js_messages.dart b/pkg/dart_messages/test/smoke_messages.dart
similarity index 99%
copy from pkg/compiler/lib/src/diagnostics/dart2js_messages.dart
copy to pkg/dart_messages/test/smoke_messages.dart
index 0f4e6247c53531ed8f8431ea22ce6300da54742b..d2113c58634e62c1a6ee080ef20b291a129850d8 100644
--- a/pkg/compiler/lib/src/diagnostics/dart2js_messages.dart
+++ b/pkg/dart_messages/test/smoke_messages.dart
@@ -58,21 +58,7 @@
 // 1. what is wrong, 2. why is it wrong, 3. how do I fix it. However, we
 // combine the first two in [template] and the last in [howToFix].
 
-/// Padding used before and between import chains in the message for
-/// [MessageKind.IMPORT_EXPERIMENTAL_MIRRORS].
-const String IMPORT_EXPERIMENTAL_MIRRORS_PADDING = '\n*   ';
-
-/// Padding used before and between import chains in the message for
-/// [MessageKind.MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND].
-const String MIRRORS_NOT_SUPPORTED_BY_BACKEND_PADDING = '\n   ';
-
-/// Padding used before and between import chains in the message for
-/// [MessageKind.DISALLOWED_LIBRARY_IMPORT].
-const String DISALLOWED_LIBRARY_IMPORT_PADDING = '\n  ';
-
-const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
-
-final Map<String, Map> MESSAGES = {
+final MESSAGES = {
   /// Do not use this. It is here for legacy and debugging. It violates item
   /// 4 of the guide lines for error messages in the beginning of the file.
   'GENERIC': {'id': 'SOWPSL', 'template': "#{text}",},