Added support for flushable functions + clarified finalValue().

utils/Stream.js

@@ -23,9 +23,15 @@ class Stream extends Duplex {
     );
   }
   _final(callback) {
-    // TODO: add isFlushable()
-    this.push(null);
-    callback(null);
+    if (!defs.isFlushable(this._fn)) {
+      this.push(null);
+      callback(null);
+      return;
+    }
+    this._processChunk(defs.none, null).then(
+      () => callback(null),
+      error => callback(error)
+    );
   }
   _read() {
     this._resume();
@@ -83,9 +89,9 @@ class Stream extends Duplex {
     }
 
     if (defs.isFinalValue(value)) {
+      // a final value is not supported, it is treated as a regular value
       value = defs.getFinalValue(value);
-      await this._processValue(value);
-      throw new defs.Stop(); // is it the correct handling of the final value?
+      return this._processValue(value);
     }
 
     if (!this.push(value)) {
@@ -100,7 +106,6 @@ class Stream extends Duplex {
       if (error instanceof defs.Stop) {
         this.push(null);
         this.destroy();
-        // clean up
         return;
       }
       throw error;