Support user generated custom native JS classes.

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 99aa824372b3e97eedd8a899eb49cf0fba56393f..b4ab696afa17380c2c4fdf565f36f0b7f9ee6a76 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -270,6 +270,8 @@ enum MessageKind {
   INVALID_UNNAMED_CONSTRUCTOR_NAME,
   INVALID_URI,
   INVALID_USE_OF_SUPER,
+  JS_INTEROP_CLASS_CANNOT_EXTEND_DART_CLASS,
+  JS_INTEROP_CLASS_NON_EXTERNAL_MEMBER,
   LIBRARY_NAME_MISMATCH,
   LIBRARY_NOT_FOUND,
   LIBRARY_TAG_MUST_BE_FIRST,
@@ -2083,6 +2085,33 @@ main() => A.A = 1;
         const MessageTemplate(MessageKind.INTERNAL_LIBRARY,
           "Internal library '#{resolvedUri}' is not accessible."),
 
+      MessageKind.JS_INTEROP_CLASS_CANNOT_EXTEND_DART_CLASS:
+        const MessageTemplate(
+          MessageKind.JS_INTEROP_CLASS_CANNOT_EXTEND_DART_CLASS,
+          "#{cls} cannot extend a normal Dart class as it is annoted with"
+          "@Js.",
+          howToFix: "Add @Js annotation to super class",
+          examples: const [
+          """
+        class Foo { }
+
+        @Js()
+        class Bar extends Foo { }
+        """]),
+
+      MessageKind.JS_INTEROP_CLASS_NON_EXTERNAL_MEMBER:
+        const MessageTemplate(
+          MessageKind.JS_INTEROP_CLASS_NON_EXTERNAL_MEMBER,
+          "Js interop class #{cls} member #{member} is not external.",
+          howToFix: "Mark all interop methods external",
+          examples: const [
+          """
+            @Js
+            class Foo {
+              external bar();
+            }
+            """]),
+
       MessageKind.LIBRARY_NOT_FOUND:
         const MessageTemplate(MessageKind.LIBRARY_NOT_FOUND,
           "Library not found '#{resolvedUri}'."),