Revert "Remove Sink and move CollectionSink to async."

sdk/lib/_internal/compiler/implementation/compiler.dart

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/compiler.dart b/sdk/lib/_internal/compiler/implementation/compiler.dart
index 873b99b81b354351e53651adb0e6646325ae30ee..6bf404ec8a796e7711e5351d223579a5f944cbe0 100644
--- a/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -1107,15 +1107,13 @@ bool invariant(Spannable spannable, var condition, {String message: null}) {
 }
 
 /// A sink that drains into /dev/null.
-class NullSink extends StreamSink<String> {
+class NullSink extends Sink<String> {
   final String name;
 
   NullSink(this.name);
 
   add(String value) {}
 
-  void signalError(AsyncError error) {}
-
   void close() {}
 
   toString() => name;