Move the status file parser into its own package.

pkg/status_file/lib/environment.dart

+// Copyright (c) 2017, 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.
+
+/// An implementation of this defines the variables that are available for use
+/// inside a status file section header.
+abstract class Environment {
+  /// Validates that the variable with [name] exists and can be compared
+  /// against [value].
+  ///
+  /// If any errors are found, adds them to [errors].
+  void validate(String name, String value, List<String> errors);
+
+  /// Looks up the value of the variable with [name].
+  String lookUp(String name);
+}