Roll Observatory packages and add a roll script

packages/which/test/which_test.dart

+
+library which.test.which;
+
+import 'package:path/path.dart';
+import 'package:unittest/unittest.dart';
+import 'package:which/which.dart';
+
+main() {
+  group('which', () {
+    // Any dart:io supported platform (*nix, osx, windows) should have `find`.
+    test('should find `find`', () => which('find').then(_testResult));
+  });
+
+  group('whichSync', () {
+    // Any dart:io supported platform (*nix, osx, windows) should have `find`.
+    test('should find `find`', () {
+      _testResult(whichSync('find'));
+    });
+  });
+}
+
+_testResult(String path) {
+  expect(path, isNotNull);
+  var base = basenameWithoutExtension(path);
+  expect(base, 'find');
+}