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

pkg/barback/test/package_graph/lazy_transformer_test.dart

Index: pkg/barback/test/package_graph/lazy_transformer_test.dart
diff --git a/pkg/barback/test/package_graph/lazy_transformer_test.dart b/pkg/barback/test/package_graph/lazy_transformer_test.dart
index 0608090ffb50ae360697e042354a15aacb21c3a8..29bbac060f9106efd105a5e50b073425f8f5aed8 100644
--- a/pkg/barback/test/package_graph/lazy_transformer_test.dart
+++ b/pkg/barback/test/package_graph/lazy_transformer_test.dart
@@ -164,8 +164,7 @@ main() {
     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]]});
 
@@ -179,7 +178,25 @@ main() {
     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 "