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

sdk/lib/_internal/pub_generated/lib/src/global_packages.dart

 library pub.global_packages;
 import 'dart:async';
 import 'dart:io';
 import 'package:path/path.dart' as p;
 import 'package:barback/barback.dart';
 import 'barback/asset_environment.dart';
 import 'entrypoint.dart';
 import 'executable.dart' as exe;
 import 'io.dart';
 import 'lock_file.dart';
@@ skipping 208 matching lines @@
     return completer0.future;
   }
   Future<Map<String, String>> _precompileExecutables(Entrypoint entrypoint,
       String package) {
     return log.progress("Precompiling executables", () {
       final completer0 = new Completer();
       scheduleMicrotask(() {
         try {
           var binDir = p.join(_directory, package, 'bin');
           cleanDir(binDir);
-          AssetEnvironment.create(
-              entrypoint,
-              BarbackMode.RELEASE,
-              useDart2JS: false).then((x0) {
+          entrypoint.loadPackageGraph().then((x0) {
             try {
-              var environment = x0;
-              environment.barback.errors.listen(((error) {
-                log.error(log.red("Build error:\n$error"));
-              }));
-              completer0.complete(
-                  environment.precompileExecutables(package, binDir));
+              var graph = x0;
+              AssetEnvironment.create(
+                  entrypoint,
+                  BarbackMode.RELEASE,
+                  entrypoints: graph.packages[package].executableIds,
+                  useDart2JS: false).then((x1) {
+                try {
+                  var environment = x1;
+                  environment.barback.errors.listen(((error) {
+                    log.error(log.red("Build error:\n$error"));
+                  }));
+                  completer0.complete(
+                      environment.precompileExecutables(package, binDir));
+                } catch (e1) {
+                  completer0.completeError(e1);
+                }
+              }, onError: (e2) {
+                completer0.completeError(e2);
+              });
             } catch (e0) {
               completer0.completeError(e0);
             }
-          }, onError: (e1) {
-            completer0.completeError(e1);
+          }, onError: (e3) {
+            completer0.completeError(e3);
           });
-        } catch (e2) {
-          completer0.completeError(e2);
+        } catch (e4) {
+          completer0.completeError(e4);
         }
       });
       return completer0.future;
     });
   }
   Future<PackageId> _cacheDependency(PackageId id) {
     final completer0 = new Completer();
     scheduleMicrotask(() {
       try {
         var source = cache.sources[id.source];
@@ skipping 288 matching lines @@
         log.fine("Could not parse binstub $file:\n$contents");
         continue;
       }
       if (match[1] == package) {
         log.fine("Deleting old binstub $file");
         deleteEntry(file);
       }
     }
   }
 }