Refactor handling of library prefixes.

pkg/compiler/lib/src/warnings.dart

Index: pkg/compiler/lib/src/warnings.dart
diff --git a/pkg/compiler/lib/src/warnings.dart b/pkg/compiler/lib/src/warnings.dart
index f14426292f4082c1e29bfdf0c98fb825e2b2afd0..49980b782e7f103fe25454089accfa0bb0a274ec 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -401,6 +401,9 @@ main() {}""",
   static const MessageKind NOT_A_PREFIX = const MessageKind(
       "'#{node}' is not a prefix.");
 
+  static const MessageKind PREFIX_AS_EXPRESSION = const MessageKind(
+      "Library prefix '#{prefix}' is not a valid expression.");
+
   static const MessageKind CANNOT_FIND_CONSTRUCTOR = const MessageKind(
       "Cannot find constructor '#{constructorName}'.");