Don't load transformers that aren't going to be used for an executable.

sdk/lib/_internal/pub_generated/test/dependency_computer/utils.dart

 library pub_tests;
+import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/scheduled_test.dart';
 import '../../lib/src/barback/cycle_exception.dart';
-import '../../lib/src/barback/transformers_needed_by_transformers.dart';
+import '../../lib/src/barback/dependency_computer.dart';
 import '../../lib/src/entrypoint.dart';
 import '../../lib/src/io.dart';
 import '../../lib/src/package.dart';
 import '../../lib/src/package_graph.dart';
 import '../../lib/src/source/path.dart';
 import '../../lib/src/system_cache.dart';
 import '../../lib/src/utils.dart';
 import '../test_pub.dart';
 void expectDependencies(Map<String, Iterable<String>> expected) {
   expected = mapMap(expected, value: (_, ids) => ids.toSet());
   schedule(() {
+    var computer = new DependencyComputer(_loadPackageGraph());
     var result = mapMap(
-        computeTransformersNeededByTransformers(_loadPackageGraph()),
+        computer.transformersNeededByTransformers(),
         key: (id, _) => id.toString(),
         value: (_, ids) => ids.map((id) => id.toString()).toSet());
     expect(result, equals(expected));
   }, "expect dependencies to match $expected");
 }
 void expectException(matcher) {
   schedule(() {
-    expect(
-        () => computeTransformersNeededByTransformers(_loadPackageGraph()),
-        throwsA(matcher));
+    expect(() {
+      var computer = new DependencyComputer(_loadPackageGraph());
+      computer.transformersNeededByTransformers();
+    }, throwsA(matcher));
   }, "expect an exception: $matcher");
 }
 void expectCycleException(Iterable<String> steps) {
   expectException(predicate((error) {
     expect(error, new isInstanceOf<CycleException>());
     expect(error.steps, equals(steps));
     return true;
   }, "cycle exception:\n${steps.map((step) => "  $step").join("\n")}"));
 }
+void expectLibraryDependencies(String id, Iterable<String> expected) {
+  expected = expected.toSet();
+  schedule(() {
+    var computer = new DependencyComputer(_loadPackageGraph());
+    var result = computer.transformersNeededByLibrary(
+        new AssetId.parse(id)).map((id) => id.toString()).toSet();
+    expect(result, equals(expected));
+  }, "expect dependencies to match $expected");
+}
 PackageGraph _loadPackageGraph() {
   var packages = {};
   var systemCache = new SystemCache(p.join(sandboxDir, cachePath));
   systemCache.sources
       ..register(new PathSource())
       ..setDefault('path');
   var entrypoint = new Entrypoint(p.join(sandboxDir, appPath), systemCache);
   for (var package in listDir(sandboxDir)) {
     if (!fileExists(p.join(package, 'pubspec.yaml'))) continue;
     var packageName = p.basename(package);
@@ skipping 21 matching lines @@
     buffer.writeln('import "$import";');
   }
   buffer.writeln("""
 NoOpTransformer extends Transformer {
   bool isPrimary(AssetId id) => true;
   void apply(Transform transform) {}
 }
 """);
   return buffer.toString();
 }