[browser] restore previous version with tighter SDK lower bound

pkg/browser/README.md

Index: pkg/browser/README.md
diff --git a/pkg/browser/README.md b/pkg/browser/README.md
index 32c736c0c6a55a603f454fa75b89446c08e5f69a..fa968513d98e59bf061ff5aad36a4ffe5b5b70d0 100644
--- a/pkg/browser/README.md
+++ b/pkg/browser/README.md
@@ -1,3 +1,8 @@
+This package contains dart.js, and previously contained interop.js
+
+dart.js
+=======
+
 The dart.js file is used in Dart browser apps to check for native Dart support
 and either (a) bootstrap Dartium or (b) load compiled JS instead.  Previously,
 we've recommended that you add a script tag pointing the version of dart.js in
@@ -20,3 +25,15 @@ If you do not wish to use pub, you may host a copy of this file locally instead.
 In this case, you will need to update it yourself as necessary.  We reserve the
 right to move the old file in the repository, so we no longer recommend linking
 to it directly.
+
+interop.js
+==========
+
+This script was required for dart:js interop to work, but it is no longer
+needed. The functionality is now supported by dart:js directly.
+
+If you previously had a script such as this, please remove it:
+
+```html
+<script src="packages/browser/interop.js"></script>
+```