Support static/global field initializations.

frog/leg/warnings.dart

Index: frog/leg/warnings.dart
diff --git a/frog/leg/warnings.dart b/frog/leg/warnings.dart
index e54f8f939ab1315c0609e4c7e31566b2facdad24..ccf8d90022aa7f743c1efcad0a4775d74af50436 100644
--- a/frog/leg/warnings.dart
+++ b/frog/leg/warnings.dart
@@ -65,6 +65,9 @@ class MessageKind {
   static final INVALID_FOR_IN = const MessageKind(
       'Invalid for-in variable declaration.');
 
+  static final NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
+      '#{1} cannot be used as compile-time constant.');
+
   toString() => template;
 }
 
@@ -115,3 +118,11 @@ class ResolutionWarning {
     : message = new Message(kind, arguments);
   String toString() => message.toString();
 }
+
+class CompileTimeConstantError {
+  final Message message;
+  CompileTimeConstantError.message(this.message);
+  CompileTimeConstantError(MessageKind kind, List<Type> arguments)
+    : message = new Message(kind, arguments);
+  String toString() => message.toString();  
+}