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 347b25b54cd2b88e1154884f07d4e5ae2191dc7d..cde6aa80a6cf1cfa4dc06881199cb13b9085d464 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -275,6 +275,8 @@ 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_NOT_SUPPORTED,
@@ -2130,6 +2132,33 @@ 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"

[Siggi Cherem (dart-lang), 2015/10/06 22:38:01]

nit: "#{cls} ... " => "Class '#{cls}' ..."

or:

  'Js-interop class '#{cls}' cannot extend from the non js-interop class
'#{superclass}'.

[Jacob, 2015/10/13 01:19:22]

much better. done.

+          "@Js.",
+          howToFix: "Add @Js annotation to super class",

[Siggi Cherem (dart-lang), 2015/10/06 22:38:01]

nits:

1.   add => add the
     super class => superclass

   or change the sentence to:

     Annotate the superclass with @Js.

2. end with a "."

[Jacob, 2015/10/13 01:19:22]

Done.

+          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.",

[Siggi Cherem (dart-lang), 2015/10/06 22:38:01]

nit: maybe change to:
   "Member '#{member}' in js-interop class '#{cls}' is not external."

(minor grammar change, add ' ' around names, end with . )

[Jacob, 2015/10/13 01:19:22]

Done.

+          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}'."),