[browser] restore previous version with tighter SDK lower bound

pkg/browser/lib/interop.js

Index: pkg/browser/lib/interop.js
diff --git a/pkg/browser/lib/interop.js b/pkg/browser/lib/interop.js
index d7c7de609292c8b55215268d380e23ffc6a76380..ec02e5857ad195d2df1c15a768f83fda4b7fa098 100644
--- a/pkg/browser/lib/interop.js
+++ b/pkg/browser/lib/interop.js
@@ -2,9 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// Type for remote proxies to Dart objects with dart2js.
-// WARNING: do not call this constructor or rely on it being
-// in the global namespace, as it may be removed.
-function DartObject(o) {
-  this.o = o;
+// TODO(jmesserly): remove this script after a deprecation period.
+if (typeof console == "object" && typeof console.warn == "function") {
+  console.warn('<script src="packages/browser/interop.js"> is no longer ' +
+      'needed for dart:js. See http://pub.dartlang.org/packages/browser.');
 }