Add prefix constraints for deferred imports.

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 014e40a20aaaa6b944ccf21ab7680febd4a2d820..23de601fdf9b3018359c8432198a0695aaba94bb 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -1201,6 +1201,18 @@ main() => A.A = 1;
             "Try adding a '@DeferredLibrary(...)' annotation in the main "
             "library");
 
+  static const MessageKind DEFERRED_LIBRARY_WITHOUT_PREFIX =
+      const MessageKind(
+          "Error: This import is deferred but there is no prefix keyword.",
+          howToFix:
+            "Try adding a prefix to the import.");
+
+  static const MessageKind DEFERRED_LIBRARY_DUPLICATE_PREFIX =
+      const MessageKind(
+          "Error: The prefix of this deferred import is not unique.",
+          howToFix:
+            "Try changing the import prefix.");
+
   static const MessageKind ILLEGAL_STATIC = const MessageKind(
       "Error: Modifier static is only allowed on functions declared in "
       "a class.");