Constrain type annotations with deferred types.

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 9b923d10e410833c13edf6121ec79ec5c81b0ad5..9e1206414088891f23893123a9fb608132183dc2 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -1160,6 +1160,11 @@ main() => A.A = 1;
             "Try adding a '@DeferredLibrary(...)' annotation in the main "
             "library");
 
+  static const MessageKind DEFERRED_LIBRARY_DART_2_DART =
+      const MessageKind(
+          "Deferred loading is not supported by the dart backend yet. "
+          "Will not split the output.");
+
   static const MessageKind DEFERRED_LIBRARY_WITHOUT_PREFIX =
       const MessageKind(
           "This import is deferred but there is no prefix keyword.",
@@ -1172,6 +1177,13 @@ main() => A.A = 1;
           howToFix:
             "Try changing the import prefix.");
 
+  static const MessageKind DEFERRED_TYPE_ANNOTATION =
+      const MessageKind(
+          "The type #{node} is deferred. "
+          "Deferred types are not valid as type annotations.",
+          howToFix:
+            "Try using a non-deferred abstract class as an interface.");
+
   static const MessageKind ILLEGAL_STATIC = const MessageKind(
       "Modifier static is only allowed on functions declared in "
       "a class.");