Fasta infrastructure.

pkg/fasta/lib/src/environment_variable.dart

Index: pkg/fasta/lib/src/environment_variable.dart
diff --git a/pkg/fasta/lib/src/environment_variable.dart b/pkg/fasta/lib/src/environment_variable.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c58ab1771ebcb3abb30052fd238106cde485bbce
--- /dev/null
+++ b/pkg/fasta/lib/src/environment_variable.dart
@@ -0,0 +1,72 @@
+// 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.
+
+library fasta.environment_variable;
+
+import 'dart:async' show
+    Future;
+
+import 'dart:io' show
+    Directory,
+    File,
+    Platform;
+
+import 'errors.dart' show
+    inputError;
+
+class EnvironmentVariable {
+  final String name;
+
+  final String what;
+
+  const EnvironmentVariable(this.name, this.what);
+
+  Future<String> get value async {
+    String value = Platform.environment[name];
+    if (value == null) return variableNotDefined();
+    await validate(value);
+    return value;
+  }
+
+  Future<Null> validate(String value) => new Future<Null>.value();
+
+  variableNotDefined() {
+    inputError(null, null,
+        "The environment variable '$name' isn't defined. $what");
+  }
+}
+
+class EnvironmentVariableFile extends EnvironmentVariable {
+  const EnvironmentVariableFile(String name, String what)
+      : super(name, what);
+
+  Future<Null> validate(String value) async {
+    if (!await new File(value).exists()) notFound(value);
+    return null;
+  }
+
+  notFound(String value) {
+    inputError(null, null,"The environment variable '$name' has the value "
+        "'$value', that isn't a file. $what");
+  }
+}
+
+class EnvironmentVariableDirectory extends EnvironmentVariable {
+  const EnvironmentVariableDirectory(String name, String what)
+      : super(name, what);
+
+  Future<Null> validate(String value) async {
+    if (!await new Directory(value).exists()) notFound(value);
+    return null;
+  }
+
+  notFound(String value) {
+    inputError(null, null, "The environment variable '$name' has the value "
+        "'$value', that isn't a directory. $what");
+  }
+}
+
+Future<bool> fileExists(Uri base, String path) async {
+  return await new File.fromUri(base.resolve(path)).exists();
+}