Isolates: allow sending of arbitrary objects in dart2js.

sdk/lib/_internal/compiler/js_lib/js_helper.dart

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 2de11198c0b566024fabc4e4135d624ee2fd2d9a..ec2958d9107749b48c9dc8ecde766c2f3dfef92a 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -77,6 +77,12 @@ class _Patch {
 
 const _Patch patch = const _Patch();
 
+
+/// Marks the internal map in dart2js, so that internal libraries can is-check
+// them.
+abstract class InternalMap {
+}
+
 /// No-op method that is called to inform the compiler that preambles might
 /// be needed when executing the resulting JS file in a command-line
 /// JS engine.