Make Platform.environment case-insensitive on Windows.

sdk/lib/io/platform.dart

Index: sdk/lib/io/platform.dart
diff --git a/sdk/lib/io/platform.dart b/sdk/lib/io/platform.dart
index 88f5f468b7a52a30d39fb6a8728f39cc51f675dc..156238a2a0df14a722d41a83df73acaaa7d85d88 100644
--- a/sdk/lib/io/platform.dart
+++ b/sdk/lib/io/platform.dart
@@ -33,6 +33,11 @@ class Platform {
 
   /**
    * Get the environment for this process.
+   *
+   * Environment variables on Windows are case-insensitive. The map
+   * returned on Windows is therefore case-insensitive and will convert
+   * all keys to upper case. On other platforms the returned map is
+   * a standard case-sensitive map.
    */
   static Map<String, String> get environment => _Platform.environment;
 }