Roll Observatory packages and add a roll script

packages/which/test/candidate_paths_test.dart

+
+library which.test.candidate_paths;
+
+import 'package:unittest/unittest.dart';
+
+import 'util.dart';
+
+main() {
+  group('getCandidatePaths', () {
+    test('posix', () {
+      var candidatePaths = getPosixCandidatePaths('z', '/x/y:/a/b/c', '/foo/bar');
+      expect(candidatePaths, ['/x/y/z', '/a/b/c/z']);
+    });
+
+    test('windows', () {
+      var candidatePaths = getWindowsCandidatePaths('z', r'C:\x\y;C:\a\b\c', '.EXE;.BAT', r'C:\foo\bar');
+      expect(candidatePaths, [
+        r'C:\foo\bar\z.EXE',
+        r'C:\foo\bar\z.BAT',
+        r'C:\x\y\z.EXE',
+        r'C:\x\y\z.BAT',
+        r'C:\a\b\c\z.EXE',
+        r'C:\a\b\c\z.BAT']);
+    });
+  });
+}