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

sdk/lib/_internal/pub_generated/test/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();
+  withBarbackVersions("any", () {
+    integration('runs a local script with customizable modes', () {
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "transformers": ["myapp/src/transformer"]
+        }),
+            d.dir(
+                "lib",
+                [
+                    d.dir(
+                        "src",
+                        [
+                            d.file("transformer.dart", TRANSFORMER),
+                            d.file("primary.in", "")])])]).create();
+      createLockFile('myapp', pkg: ['barback']);
+      var pub = pubRun(args: ["script"]);
+      pub.stdout.expect("debug");
+      pub.shouldExit();
+      pub = pubRun(args: ["--mode", "custom-mode", "script"]);
+      pub.stdout.expect("custom-mode");
+      pub.shouldExit();
+    });
+    integration('runs a dependency script with customizable modes', () {
+      d.dir("foo", [d.pubspec({
+          "name": "foo",
+          "version": "1.2.3",
+          "transformers": ["foo/src/transformer"]
+        }),
+            d.dir(
+                "lib",
+                [
+                    d.dir(
+                        "src",
+                        [
+                            d.file("transformer.dart", TRANSFORMER),
+                            d.file("primary.in", "")])])]).create();
+      d.appDir({
+        "foo": {
+          "path": "../foo"
+        }
+      }).create();
+      createLockFile('myapp', sandbox: ['foo'], pkg: ['barback']);
+      var pub = pubRun(args: ["foo:script"]);
+      pub.stdout.expect("release");
+      pub.shouldExit();
+      pub = pubRun(args: ["--mode", "custom-mode", "foo:script"]);
+      pub.stdout.expect("custom-mode");
+      pub.shouldExit();
+    });
+  });
+}