Add an AssetStream class to Barback.

pkg/barback/test/asset_test.dart

Index: pkg/barback/test/asset_test.dart
diff --git a/pkg/barback/test/asset_test.dart b/pkg/barback/test/asset_test.dart
index b26d4629cde8142236d4987b44c8dda43ddcdd0d..05ece1fe810ee1df9749889da82c49fac2306827 100644
--- a/pkg/barback/test/asset_test.dart
+++ b/pkg/barback/test/asset_test.dart
@@ -73,6 +73,14 @@ main() {
     });
   });
 
+  group("Asset.fromStream", () {
+    test("returns an asset with the given ID", () {
+      var asset = new Asset.fromStream(id,
+          new Stream.fromFuture(new Future.value([104, 101, 108, 108, 111])));
+      expect(asset.id, equals(id));
+    });
+  });
+
   group("read()", () {
     test("gets the UTF-8-encoded string for a string asset", () {
       var asset = new Asset.fromString(id, "çøñ†éℵ™");
@@ -97,6 +105,13 @@ main() {
       expect(asset.read().toList(),
           completion(equals([encodeUtf8("çøñ†éℵ™")])));
     });
+
+    test("gets the raw bytes for a stream", () {
+      var asset = new Asset.fromStream(id,
+          new Stream.fromFuture(new Future.value(encodeUtf8("çøñ†éℵ™"))));
+      expect(asset.read().toList(),
+          completion(equals([encodeUtf8("çøñ†éℵ™")])));
+    });
   });
 
   group("readAsString()", () {
@@ -137,6 +152,24 @@ main() {
             completion(equals("çøñ†éℵ™")));
       });
     });
+
+    group("stream asset", () {
+      test("defaults to UTF-8 if encoding is omitted", () {
+        var asset = new Asset.fromStream(id,
+            new Stream.fromFuture(new Future.value(encodeUtf8("çøñ†éℵ™"))));
+        expect(asset.readAsString(),
+            completion(equals("çøñ†éℵ™")));
+      });
+
+      test("supports UTF-8", () {
+        var asset = new Asset.fromStream(id,
+            new Stream.fromFuture(new Future.value(encodeUtf8("çøñ†éℵ™"))));
+        expect(asset.readAsString(encoding: UTF8),
+            completion(equals("çøñ†éℵ™")));
+      });
+
+      // TODO(nweiz): Test other encodings once #6284 is fixed.
+    });
   });
 
   group("toString()", () {