Add a --mode flag to pub run and pub global run.

sdk/lib/_internal/pub_generated/test/global/run/mode_test.dart

+import '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+const TRANSFORMER = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class DartTransformer extends Transformer {
+  final BarbackSettings _settings;
+
+  DartTransformer.asPlugin(this._settings);
+
+  String get allowedExtensions => '.in';
+
+  void apply(Transform transform) {
+    transform.addOutput(new Asset.fromString(
+        new AssetId(transform.primaryInput.id.package, "bin/script.dart"),
+        "void main() => print('\${_settings.mode.name}');"));
+  }
+}
+""";
+main() {
+  initConfig();
+  integration(
+      'runs a script in an activated package with customizable modes',
+      () {
+    servePackages((builder) {
+      builder.serveRepoPackage("barback");
+      builder.serve("foo", "1.0.0", deps: {
+        "barback": "any"
+      }, pubspec: {
+        "transformers": ["foo/src/transformer"]
+      },
+          contents: [
+              d.dir(
+                  "lib",
+                  [
+                      d.dir(
+                          "src",
+                          [d.file("transformer.dart", TRANSFORMER), d.file("primary.in", "")])])]);
+    });
+    schedulePub(args: ["global", "activate", "foo"]);
+    var pub = pubRun(global: true, args: ["foo:script"]);
+    pub.stdout.expect("release");
+    pub.shouldExit();
+    pub = pubRun(global: true, args: ["--mode", "custom-mode", "foo:script"]);
+    pub.stdout.expect("custom-mode");
+    pub.shouldExit();
+  });
+}