Add Capability implementation to dart2js libraries.

sdk/lib/_internal/lib/isolate_patch.dart

Index: sdk/lib/_internal/lib/isolate_patch.dart
diff --git a/sdk/lib/_internal/lib/isolate_patch.dart b/sdk/lib/_internal/lib/isolate_patch.dart
index 4ad59ca943fd54b364e716ca3dac97479a2360eb..09b79553abb1d1787346d0d78d9af8f98e5d8e9b 100644
--- a/sdk/lib/_internal/lib/isolate_patch.dart
+++ b/sdk/lib/_internal/lib/isolate_patch.dart
@@ -4,11 +4,12 @@
 
 // Patch file for the dart:isolate library.
 
-import 'dart:_isolate_helper' show IsolateNatives,
-                                   ReceivePortImpl,
-                                   RawReceivePortImpl,
+import 'dart:_isolate_helper' show CapabilityImpl,
                                    CloseToken,
-                                   JsIsolateSink;
+                                   IsolateNatives,
+                                   JsIsolateSink,
+                                   ReceivePortImpl,
+                                   RawReceivePortImpl;
 
 patch class Isolate {
   patch static Future<Isolate> spawn(void entryPoint(message), var message) {
@@ -56,7 +57,5 @@ patch class RawReceivePort {
 }
 
 patch class Capability {
-  patch factory Capability() {
-    throw new UnimplementedError();
-  }
+  patch factory Capability() = CapabilityImpl;
 }