Refactor the barback tests somewhat.

pkg/barback/test/asset_set_test.dart

Index: pkg/barback/test/asset_set_test.dart
diff --git a/pkg/barback/test/asset_set_test.dart b/pkg/barback/test/asset_set_test.dart
index 6cb1eb8385814636cf3d0b69f154c9fe218399b5..5b6b652478f624a8811d9724ecc12eb1e80fcece 100644
--- a/pkg/barback/test/asset_set_test.dart
+++ b/pkg/barback/test/asset_set_test.dart
@@ -18,11 +18,25 @@ main() {
 
   var fooId = new AssetId.parse("app|foo.txt");
   var barId = new AssetId.parse("app|bar.txt");
+  var bazId = new AssetId.parse("app|baz.txt");
+
+  group(".from()", () {
+    test("creates a set from an iterable", () {
+      var set = new AssetSet.from([
+        new Asset.fromString(fooId, "foo"),
+        new Asset.fromString(barId, "bar")
+      ]);
+
+      expect(set.containsId(fooId), isTrue);
+      expect(set.containsId(barId), isTrue);
+      expect(set.containsId(bazId), isFalse);
+    });
+  });
 
   group("[] operator", () {
     test("gets an asset with the given ID", () {
       var set = new AssetSet();
-      var foo = new MockAsset(fooId, "foo");
+      var foo = new Asset.fromString(fooId, "foo");
       set.add(foo);
 
       expect(set[fooId], equals(foo));
@@ -37,21 +51,21 @@ main() {
   group(".add()", () {
     test("adds the asset to the set", () {
       var set = new AssetSet();
-      var foo = new MockAsset(fooId, "foo");
+      var foo = new Asset.fromString(fooId, "foo");
       set.add(foo);
       expect(set.contains(foo), isTrue);
     });
 
     test("replaces a previously added asset with that ID", () {
       var set = new AssetSet();
-      set.add(new MockAsset(fooId, "before"));
-      set.add(new MockAsset(fooId, "after"));
-      expect(set[fooId].contents, equals("after"));
+      set.add(new Asset.fromString(fooId, "before"));
+      set.add(new Asset.fromString(fooId, "after"));
+      expect(set[fooId].readAsString(), completion(equals("after")));
     });
 
     test("returns the added item", () {
       var set = new AssetSet();
-      var foo = new MockAsset(fooId, "foo");
+      var foo = new Asset.fromString(fooId, "foo");
       expect(set.add(foo), equals(foo));
     });
   });
@@ -59,8 +73,8 @@ main() {
   group(".addAll()", () {
     test("adds the assets to the set", () {
       var set = new AssetSet();
-      var foo = new MockAsset(fooId, "foo");
-      var bar = new MockAsset(barId, "bar");
+      var foo = new Asset.fromString(fooId, "foo");
+      var bar = new Asset.fromString(barId, "bar");
       set.addAll([foo, bar]);
       expect(set.contains(foo), isTrue);
       expect(set.contains(bar), isTrue);
@@ -68,17 +82,17 @@ main() {
 
     test("replaces assets earlier in the sequence with later ones", () {
       var set = new AssetSet();
-      var foo1 = new MockAsset(fooId, "before");
-      var foo2 = new MockAsset(fooId, "after");
+      var foo1 = new Asset.fromString(fooId, "before");
+      var foo2 = new Asset.fromString(fooId, "after");
       set.addAll([foo1, foo2]);
-      expect(set[fooId].contents, equals("after"));
+      expect(set[fooId].readAsString(), completion(equals("after")));
     });
   });
 
   group(".clear()", () {
     test("empties the set", () {
       var set = new AssetSet();
-      var foo = new MockAsset(fooId, "foo");
+      var foo = new Asset.fromString(fooId, "foo");
       set.add(foo);
       set.clear();
 
@@ -90,8 +104,8 @@ main() {
   group(".contains()", () {
     test("returns true if the asset is in the set", () {
       var set = new AssetSet();
-      var foo = new MockAsset(fooId, "foo");
-      var bar = new MockAsset(barId, "bar");
+      var foo = new Asset.fromString(fooId, "foo");
+      var bar = new Asset.fromString(barId, "bar");
       set.add(foo);
 
       expect(set.contains(foo), isTrue);
@@ -102,11 +116,38 @@ main() {
   group(".containsId()", () {
     test("returns true if an asset with the ID is in the set", () {
       var set = new AssetSet();
-      var foo = new MockAsset(fooId, "foo");
+      var foo = new Asset.fromString(fooId, "foo");
       set.add(foo);
 
       expect(set.containsId(fooId), isTrue);
       expect(set.containsId(barId), isFalse);
     });
   });
+
+  group(".removeId()", () {
+    test("removes the asset with the ID from the set", () {
+      var set = new AssetSet();
+      var foo = new Asset.fromString(fooId, "foo");
+      set.add(foo);
+
+      set.removeId(fooId);
+      expect(set.containsId(fooId), isFalse);
+    });
+
+    test("returns the removed asset", () {
+      var set = new AssetSet();
+      var foo = new Asset.fromString(fooId, "foo");
+      set.add(foo);
+
+      expect(set.removeId(fooId).readAsString(), completion(equals("foo")));
+    });
+
+    test("returns null when removing an asset not in the set", () {
+      var set = new AssetSet();
+      var foo = new Asset.fromString(fooId, "foo");
+      set.add(foo);
+
+      expect(set.removeId(barId), isNull);
+    });
+  });
 }