Add stubbed out implementations of env. variables methods (#2405)

src/System.Private.CoreLib/src/Internal/Runtime/Augments/EnvironmentAugments.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections;
 using System.Runtime;
 using System.Runtime.CompilerServices;
 
@@ -90,5 +91,12 @@ public static string StackTrace
         }
 
         public static int TickCount => Environment.TickCount;
+
+        public static string GetEnvironmentVariable(string variable) => Environment.GetEnvironmentVariable(variable);
+        public static string GetEnvironmentVariable(string variable, EnvironmentVariableTarget target) { throw new NotImplementedException(); }
+        public static IDictionary GetEnvironmentVariables() { throw new NotImplementedException(); }
+        public static IDictionary GetEnvironmentVariables(EnvironmentVariableTarget target) { throw new NotImplementedException(); }
+        public static void SetEnvironmentVariable(string variable, string value) { throw new NotImplementedException(); }
+        public static void SetEnvironmentVariable(string variable, string value, EnvironmentVariableTarget target) { throw new NotImplementedException(); }
     }
 }

src/System.Private.CoreLib/src/System/Environment.cs

@@ -25,6 +25,13 @@
 
 namespace System
 {
+    public enum EnvironmentVariableTarget
+    {
+        Process = 0,
+        User = 1,
+        Machine = 2,
+    }
+
     // Environment is marked as Eager to allow Lock to read the current
     // thread ID, since Lock is used in ClassConstructorRunner.Cctor.GetCctor
     [EagerOrderedStaticConstructor(EagerStaticConstructorOrder.SystemEnvironment)]