Fix to dartbug.com/5170, explictly static library variables now get an appropriate error message.

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 240fe5874eadd3cc9bebe927428d088d4c658033..d2d2fb0708a26231ba6e3cb5cf63eb157f546224 100644
--- a/lib/compiler/implementation/warnings.dart
+++ b/lib/compiler/implementation/warnings.dart
@@ -297,6 +297,9 @@ Length: #{3}''');
   static const PATCH_PARAMETER_MISMATCH = const MessageKind(
       "Patch method parameter '#{3}' doesn't match '#{2}' on origin method "
       "#{1}.");
+  
+  static const TOP_LEVEL_VARIABLE_DECLARED_STATIC = const MessageKind(
+      "Top-level variable cannot be declared static.");
 
   static const COMPILER_CRASHED = const MessageKind(
       "Error: The compiler crashed when compiling this element.");