Split loader from the rest of the compiler. This adds several abstractions to

pkg/compiler/lib/src/environment.dart

+// Copyright (c) 2016, 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.
+
+/// Collects definitions provided to the compiler via the `-D` flag.
+///
+/// Environment variables can be used in the user code in two ways. From
+/// conditional imports, and from `const String.fromEnvironment` and
+/// other similar constructors.
+abstract class Environment {
+  /// Return the string value of the given key.
+  ///
+  /// Note that `bool.fromEnvironment` and `int.fromEnvironment` are also
+  /// implemented in terms of `String.fromEnvironment`.
+  String valueOf(String key);
+}