Don't crash for pre-1.12 global packages.

test/global/run/runs_script_without_packages_file_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:path/path.dart' as p;
+import 'package:pub/src/io.dart';
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+
+main() {
+  integration('runs a snapshotted script without a .packages file', () {
+    servePackages((builder) {
+      builder.serve("foo", "1.0.0", contents: [
+        d.dir("bin", [
+          d.file("script.dart", "main(args) => print('ok');")
+        ])
+      ]);
+    });
+
+    schedulePub(args: ["global", "activate", "foo"]);
+
+    // Mimic the global packages installed by pub <1.12, which didn't create a
+    // .packages file for global installs.
+    schedule(() {
+      deleteEntry(p.join(sandboxDir, cachePath,
+          'global_packages/foo/.packages'));
+    });
+
+    var pub = pubRun(global: true, args: ["foo:script"]);
+    pub.stdout.expect("ok");
+    pub.shouldExit();
+  });
+
+  integration('runs an unsnapshotted script without a .packages file', () {
+    d.dir("foo", [
+      d.libPubspec("foo", "1.0.0"),
+      d.dir("bin", [
+        d.file("foo.dart", "main() => print('ok');")
+      ])
+    ]).create();
+
+    schedulePub(args: ["global", "activate", "--source", "path", "../foo"]);
+
+    schedule(() {
+      deleteEntry(p.join(sandboxDir, cachePath,
+          'global_packages/foo/.packages'));
+    });
+
+    var pub = pubRun(global: true, args: ["foo"]);
+    pub.stdout.expect("ok");
+    pub.shouldExit();
+  });
+}