Don't make lazy transformers eager when an asset is requested.

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 146 matching lines @@
       "app|a.txt": "a.inc"
     }, {"app": [
       [new LazyManyToOneTransformer("txt")]
     ]});
 
     updateSources(["app|a.inc", "app|a.txt"]);
     expectAsset("app|a.out", "a");
     buildShouldSucceed();
   });
 
-  test("once a lazy transformer is materialized, it runs eagerly afterwards",
-      () {
+  test("after being materialized a lazy transformer is still lazy", () {
     var transformer = new LazyRewriteTransformer("blub", "blab");
     initGraph(["app|foo.blub"], {"app": [[transformer]]});
 
     updateSources(["app|foo.blub"]);
     buildShouldSucceed();
 
     // Request the asset once to force it to be materialized.
     expectAsset("app|foo.blab", "foo.blab");
     buildShouldSucceed();
 
     updateSources(["app|foo.blub"]);
     buildShouldSucceed();
 
-    expect(transformer.numRuns, completion(equals(2)));
+    expect(transformer.numRuns, completion(equals(1)));
+  });
+
+  test("after being materialized a lazy transformer can be materialized again",
+      () {
+    var transformer = new LazyRewriteTransformer("blub", "blab");
+    initGraph(["app|foo.blub"], {"app": [[transformer]]});
+
+    updateSources(["app|foo.blub"]);
+    buildShouldSucceed();
+
+    // Request the asset once to force it to be materialized.
+    expectAsset("app|foo.blab", "foo.blab");
+    buildShouldSucceed();
+
+    modifyAsset("app|foo.blub", "bar");
+    updateSources(["app|foo.blub"]);
+    expectAsset("app|foo.blab", "bar.blab");
+    buildShouldSucceed();
   });
 
   test("an error emitted in a lazy transformer's declareOutputs method is "
       "caught and reported", () {
     initGraph(["app|foo.txt"], {"app": [
       [new LazyBadTransformer("app|foo.out")]
     ]});
 
     updateSources(["app|foo.txt"]);
     buildShouldFail([isTransformerException(equals(LazyBadTransformer.ERROR))]);
@@ skipping 80 matching lines @@
     resumeProvider();
     expectAsset("app|foo.txt", "foo");
     buildShouldSucceed();
 
     modifyAsset("app|foo.txt", "bar");
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt", "bar");
     buildShouldSucceed();
   });
 }