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

sdk/lib/_internal/pub/lib/src/executable.dart

Index: sdk/lib/_internal/pub/lib/src/executable.dart
diff --git a/sdk/lib/_internal/pub/lib/src/executable.dart b/sdk/lib/_internal/pub/lib/src/executable.dart
index 2becb86920e12dc18f4914507da7c87b77d5cbef..38105925f88befd4f655776caada5d51c17d5710 100644
--- a/sdk/lib/_internal/pub/lib/src/executable.dart
+++ b/sdk/lib/_internal/pub/lib/src/executable.dart
@@ -36,6 +36,21 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
     BarbackMode mode}) async {
   if (mode == null) mode = BarbackMode.RELEASE;
 
+  // Make sure the package is an immediate dependency of the entrypoint or the
+  // entrypoint itself.
+  if (entrypoint.root.name != package &&
+      !entrypoint.root.immediateDependencies
+          .any((dep) => dep.name == package)) {
+    var graph = await entrypoint.loadPackageGraph();
+    if (graph.packages.containsKey(package)) {
+      dataError('Package "$package" is not an immediate dependency.\n'
+          'Cannot run executables in transitive dependencies.');
+    } else {
+      dataError('Could not find package "$package". Did you forget to add a '
+          'dependency?');
+    }
+  }
+
   // Unless the user overrides the verbosity, we want to filter out the
   // normal pub output shown while loading the environment.
   if (log.verbosity == log.Verbosity.NORMAL) {
@@ -69,10 +84,13 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
     executable = p.join("bin", executable);
   }
 
+  var assetPath = "${p.url.joinAll(p.split(executable))}.dart";
+  var id = new AssetId(package, assetPath);
+
   // TODO(nweiz): Use [packages] to only load assets from packages that the
   // executable might load.
   var environment = await AssetEnvironment.create(entrypoint, mode,
-      useDart2JS: false);
+      useDart2JS: false, entrypoints: [id]);
   environment.barback.errors.listen((error) {
     log.error(log.red("Build error:\n$error"));
   });
@@ -84,27 +102,10 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
     // will work from within some deeply nested script.
     server = await environment.serveDirectory(rootDir);
   } else {
-    // Make sure the dependency exists.
-    var dep = entrypoint.root.immediateDependencies.firstWhere(
-        (dep) => dep.name == package, orElse: () => null);
-    if (dep == null) {
-      if (environment.graph.packages.containsKey(package)) {
-        dataError('Package "$package" is not an immediate dependency.\n'
-            'Cannot run executables in transitive dependencies.');
-      } else {
-        dataError('Could not find package "$package". Did you forget to '
-            'add a dependency?');
-      }
-    }
-
     // For other packages, always use the "bin" directory.
     server = await environment.servePackageBinDirectory(package);
   }
 
-  // Try to make sure the entrypoint script exists (or is generated) before
-  // we spawn the process to run it.
-  var assetPath = "${p.url.joinAll(p.split(executable))}.dart";
-  var id = new AssetId(server.package, assetPath);
   // TODO(rnystrom): Use try/catch here when
   // https://github.com/dart-lang/async_await/issues/4 is fixed.
   return environment.barback.getAssetById(id).then((_) async {