Require that injected members are private.

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index ce8b6a7c0c579bca3ccf27aae87173dc00612b0d..810e88f9eb5987d2665ea407d243e04b78d09d12 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -237,6 +237,7 @@ enum MessageKind {
   INHERITED_EXPLICIT_GETTER,
   INHERITED_IMPLICIT_GETTER,
   INHERITED_METHOD,
+  INJECTED_PUBLIC_MEMBER,
   INIT_STATIC_FIELD,
   INITIALIZING_FORMAL_NOT_ALLOWED,
   INSTANCE_STATIC_SAME_NAME,
@@ -3329,6 +3330,10 @@ part of test.main;
           "Cannot patch non-function with function patch "
           "'#{functionName}'."),
 
+      MessageKind.INJECTED_PUBLIC_MEMBER:
+        const MessageTemplate(MessageKind.INJECTED_PUBLIC_MEMBER,
+            "Non-patch members in patch libraries must be private."),
+
       MessageKind.EXTERNAL_WITH_BODY:
         const MessageTemplate(MessageKind.EXTERNAL_WITH_BODY,
           "External function '#{functionName}' cannot have a function body.",