Add AngelEnv class

lib/src/core/env.dart

@@ -0,0 +1,27 @@
+import 'dart:io';
+
+/// A constant instance of [AngelEnv].
+const AngelEnv angelEnv = const AngelEnv();
+
+/// Queries the environment's `ANGEL_ENV` value.
+class AngelEnv {
+  final String _customValue;
+
+  /// You can optionally provide a custom value, in order to override the system's
+  /// value.
+  const AngelEnv([this._customValue]);
+
+  /// Returns the value of the `ANGEL_ENV` variable; defaults to `'development'`.
+  String get value =>
+      (_customValue ?? Platform.environment['ANGEL_ENV'] ?? 'development')
+          .toLowerCase();
+
+  /// Returns whether the [value] is `'development'`.
+  bool get isDevelopment => value == 'development';
+
+  /// Returns whether the [value] is `'production'`.
+  bool get isProduction => value == 'production';
+
+  /// Returns whether the [value] is `'staging'`.
+  bool get isStaging => value == 'staging';
+}