Remove support for the DeferredLibrary annotation and remove tests of it

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index 6fc0277d565069d45a5c0f36af6ecff1e33370da..95290e6562c78bdaac9f8be37875a869f0835529 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -1185,6 +1185,12 @@ main() => A.A = 1;
           howToFix:
             "Try adding a prefix to the import.");
 
+  static const MessageKind DEFERRED_OLD_SYNTAX =
+      const MessageKind(
+          "The DeferredLibrary annotation is obsolete.",
+          howToFix:
+            "Use the \"import 'lib.dart' deferred as prefix\" syntax instead.");
+
   static const MessageKind DEFERRED_LIBRARY_DUPLICATE_PREFIX =
       const MessageKind(
           "The prefix of this deferred import is not unique.",