Introduce class Sink<T>.

sdk/lib/convert/converter.dart

Index: sdk/lib/convert/converter.dart
diff --git a/sdk/lib/convert/converter.dart b/sdk/lib/convert/converter.dart
index b4c12f00225c028df203eccd190751cc0dca4011..562f861bee83fdf78cd6e508a63b017e4bcc2554 100644
--- a/sdk/lib/convert/converter.dart
+++ b/sdk/lib/convert/converter.dart
@@ -31,7 +31,7 @@ abstract class Converter<S, T> implements StreamTransformer {
   /**
    * Starts a chunked conversion.
    */
-  ChunkedConversionSink startChunkedConversion(ChunkedConversionSink sink) {
+  ChunkedConversionSink startChunkedConversion(Sink sink) {
     throw new UnsupportedError(
         "This converter does not support chunked conversions: $this");
   }
@@ -57,7 +57,7 @@ class _FusedConverter<S, M, T> extends Converter<S, T> {
 
   T convert(S input) => _second.convert(_first.convert(input));
 
-  ChunkedConversionSink startChunkedConversion(ChunkedConversionSink sink) {
+  ChunkedConversionSink startChunkedConversion(Sink sink) {
     return _first.startChunkedConversion(_second.startChunkedConversion(sink));
   }
 }