Roll Observatory packages and add a roll script

which/lib/src/candidate_paths.dart

-
-library which.src.candidate_paths;
-
-import 'dart:io';
-
-import 'package:path/path.dart';
-
-Iterable<String> getCandidatePaths(String command, Map<String, String> environment, bool isWindows, Context context) {
-  if (context.isAbsolute(command)) return [command];
-
-  String getEnvVar(String envVar, String defaultValue) {
-    var v = environment[envVar];
-    return v == null ? defaultValue : v;
-  }
-
-  var pathVarSeparator = isWindows ? ";" : ":";
-
-  List<String> splitEnvVar(String envVar, String defaultValue) =>
-      getEnvVar(envVar, defaultValue).split(pathVarSeparator);
-
-  var pathEnv = splitEnvVar('PATH', '');
-
-  var noExtPaths =
-      pathEnv.map((pathEntry) => context.join(pathEntry, command));
-
-  if (!isWindows) return noExtPaths;
-
-  pathEnv.insert(0, context.current);
-  var pathExt = splitEnvVar('PATHEXT', ".EXE");
-  if (command.contains('.')) pathExt.insert(0, '');
-  return noExtPaths.expand((commandPath) =>
-      pathExt.map((pathExtEntry) => commandPath + pathExtEntry));
-}
-
-Iterable<String> getRealCandidatePaths(String command) =>
-    getCandidatePaths(command, Platform.environment, Platform.isWindows, context);