Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/test/package_graph/lazy_transformer_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.test.package_graph.lazy_asset_test;
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
@@ skipping 64 matching lines @@
     var transformer = new LazyRewriteTransformer("blub", "blab");
     initGraph(["app|foo.blub"], {"app": [
       [transformer],
       [new RewriteTransformer("blab", "blib")]
     ]});
     updateSources(["app|foo.blub"]);
     buildShouldSucceed();
     expect(transformer.numRuns, completion(equals(1)));
   });
 
-  // TODO(nweiz): enable these once DeclaringTransformers forward laziness
-  // properly (issue 16442).
-  //
   // test("a lazy asset piped into a declaring transformer isn't eagerly "
   //     "compiled", () {
   //   var transformer1 = new LazyRewriteTransformer("blub", "blab");
   //   var transformer2 = new DeclaringRewriteTransformer("blab", "blib");
   //   initGraph(["app|foo.blub"], {"app": [
   //     [transformer1], [transformer2]
   //   ]});
   //   updateSources(["app|foo.blub"]);
   //   buildShouldSucceed();
   //   expect(transformer1.numRuns, completion(equals(0)));
@@ skipping 124 matching lines @@
     ]});
 
     updateSources(["app|foo.txt"]);
     expectNoAsset("app|foo.blab");
 
     // Getting all assets will force every lazy transformer. This shouldn't
     // cause the rewrite to apply, because foo.txt isn't primary.
     expectAllAssets(["app|foo.txt"]);
     buildShouldSucceed();
   });
+
+  // Regression test.
+  test("a lazy transformer that doesn't apply updates its passed-through asset",
+      () {
+    initGraph(["app|foo.txt"], {"app": [
+      [new LazyRewriteTransformer("blub", "blab")]
+    ]});
+
+    // Pause the provider so that the transformer will start forwarding the
+    // asset while it's dirty.
+    pauseProvider();
+    updateSources(["app|foo.txt"]);
+    expectAssetDoesNotComplete("app|foo.txt");
+
+    resumeProvider();
+    expectAsset("app|foo.txt", "foo");
+    buildShouldSucceed();
+
+    modifyAsset("app|foo.txt", "bar");
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.txt", "bar");
+    buildShouldSucceed();
+  });
 }