Barback transforms now pass through the primary input by default.

pkg/barback/test/package_graph/add_remove_transform_test.dart

Index: pkg/barback/test/package_graph/add_remove_transform_test.dart
diff --git a/pkg/barback/test/package_graph/add_remove_transform_test.dart b/pkg/barback/test/package_graph/add_remove_transform_test.dart
index ac0076cd8f1f6df3d82c75db435a69a8fc121cf8..c593907594809cb4789ea224679934687dc603a1 100644
--- a/pkg/barback/test/package_graph/add_remove_transform_test.dart
+++ b/pkg/barback/test/package_graph/add_remove_transform_test.dart
@@ -20,7 +20,6 @@ main() {
 
     updateTransformers("app", [[new RewriteTransformer("blub", "blab")]]);
     expectAsset("app|foo.blab", "foo.blab");
-    expectNoAsset("app|foo.blub");
     buildShouldSucceed();
   });
 
@@ -43,12 +42,10 @@ main() {
 
     updateSources(["app|foo.blub"]);
     expectAsset("app|foo.blab", "foo.blab");
-    expectNoAsset("app|foo.blub");
     buildShouldSucceed();
 
     updateTransformers("app", [[rewrite]]);
     expectAsset("app|foo.blab", "foo.blab");
-    expectNoAsset("app|foo.blub");
     buildShouldSucceed();
 
     expect(rewrite.numRuns, completion(equals(1)));
@@ -61,7 +58,6 @@ main() {
 
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.blab", "foo.blub.blab");
-    expectNoAsset("app|foo.blub");
     buildShouldSucceed();
 
     updateTransformers("app", [[rewrite2], [rewrite1]]);
@@ -94,7 +90,6 @@ main() {
 
     updateSources(["app|foo.blub"]);
     expectAsset("app|foo.blab", "foo.blab");
-    expectNoAsset("app|foo.blub");
     buildShouldSucceed();
 
     updateTransformers("app", []);
@@ -192,22 +187,6 @@ main() {
     buildShouldSucceed();
   });
 
-  test("a new transformer can see pass-through assets", () {
-    var rewrite = new RewriteTransformer("zip", "zap");
-    initGraph(["app|foo.blub"], {"app": [[rewrite]]});
-
-    updateSources(["app|foo.blub"]);
-    buildShouldSucceed();
-
-    updateTransformers("app", [
-      [rewrite],
-      [new RewriteTransformer("blub", "blab")]
-    ]);
-    expectAsset("app|foo.blab", "foo.blab");
-    expectNoAsset("app|foo.blub");
-    buildShouldSucceed();
-  });
-
   test("a cross-package transform sees a new transformer in a new phase", () {
     var rewrite = new RewriteTransformer("inc", "inc");
     initGraph({
@@ -253,6 +232,108 @@ main() {
     buildShouldSucceed();
   });
 
+  group("pass-through", () {
+    test("a new transformer can see pass-through assets", () {
+      var rewrite = new RewriteTransformer("zip", "zap");
+      initGraph(["app|foo.blub"], {"app": [[rewrite]]});
+
+      updateSources(["app|foo.blub"]);
+      buildShouldSucceed();
+
+      updateTransformers("app", [
+        [rewrite],
+        [new RewriteTransformer("blub", "blab")]
+      ]);
+      expectAsset("app|foo.blab", "foo.blab");
+      buildShouldSucceed();
+    });
+
+    test("a new transformer can overwrite an old asset", () {
+      var rewrite = new RewriteTransformer("zip", "zap");
+      initGraph(["app|foo.txt"], {"app": [[rewrite]]});
+
+      updateSources(["app|foo.txt"]);
+      expectAsset("app|foo.txt", "foo");
+      buildShouldSucceed();
+
+      // Add a transformer that will overwrite the previously-passed-through
+      // "foo.txt" asset. The transformed asset should be emitted, not the
+      // passed-through asset.
+      updateTransformers("app", [
+        [rewrite, new RewriteTransformer("txt", "txt")]
+      ]);
+      expectAsset("app|foo.txt", "foo.txt");
+      buildShouldSucceed();
+    });
+
+    test("passes an asset through when an overwriting transform is removed",
+        () {
+      initGraph(["app|foo.txt"], {
+        "app": [[new RewriteTransformer("txt", "txt")]]
+      });
+
+      updateSources(["app|foo.txt"]);
+      expectAsset("app|foo.txt", "foo.txt");
+      buildShouldSucceed();
+
+      updateTransformers("app", [[]]);
+      expectAsset("app|foo.txt", "foo");
+      buildShouldSucceed();
+    });
+
+    test("passes an asset through when its overwriting transform is removed "
+        "during apply", () {
+      var rewrite = new RewriteTransformer("txt", "txt");
+      initGraph(["app|foo.txt"], {"app": [[rewrite]]});
+
+      rewrite.pauseApply();
+      updateSources(["app|foo.txt"]);
+      expectAssetDoesNotComplete("app|foo.txt");
+
+      updateTransformers("app", [[]]);
+      rewrite.resumeApply();
+      expectAsset("app|foo.txt", "foo");
+      buildShouldSucceed();
+    });
+
+    test("doesn't pass an asset through when its overwriting transform is "
+        "removed during apply if another transform overwrites it", () {
+      var rewrite1 = new RewriteTransformer("txt", "txt");
+      var rewrite2 = new RewriteTransformer("txt", "txt");
+      initGraph(["app|foo.txt"], {"app": [[rewrite1, rewrite2]]});
+
+      rewrite1.pauseApply();
+      updateSources(["app|foo.txt"]);
+      expectAssetDoesNotComplete("app|foo.txt");
+      // Ensure we're waiting on [rewrite1.apply]
+      schedule(pumpEventQueue);
+
+      updateTransformers("app", [[rewrite2]]);
+      rewrite1.resumeApply();
+      expectAsset("app|foo.txt", "foo.txt");
+      buildShouldSucceed();
+    });
+
+    test("doesn't pass an asset through when one overwriting transform is "
+        "removed if another transform still overwrites it", () {
+      var rewrite = new RewriteTransformer("txt", "txt");
+      initGraph(["app|foo.txt"], {"app": [[
+        rewrite,
+        new RewriteTransformer("txt", "txt")
+      ]]});
+
+      updateSources(["app|foo.txt"]);
+      // This could be either the output of [CheckContentTransformer] or
+      // [RewriteTransformer], depending which completes first.
+      expectAsset("app|foo.txt", anything);
+      buildShouldFail([isAssetCollisionException("app|foo.txt")]);
+
+      updateTransformers("app", [[rewrite]]);
+      expectAsset("app|foo.txt", "foo.txt");
+      buildShouldSucceed();
+    });
+  });
+
   // Regression test.
   test("a phase is added, then an input is removed and re-added", () {
     var rewrite = new RewriteTransformer("txt", "mid");