Prevent creation of additional FileMode and Encoding objects

runtime/bin/input_stream.dart

Index: runtime/bin/input_stream.dart
diff --git a/runtime/bin/input_stream.dart b/runtime/bin/input_stream.dart
index 026ddc13d0aeb0396e5bb11e93c65fdc980db532..c1ca3270d14585885dabf969c165a3f69bde96d2 100644
--- a/runtime/bin/input_stream.dart
+++ b/runtime/bin/input_stream.dart
@@ -104,10 +104,10 @@ interface InputStream {
  * [StringOutputStream].
  */
 class Encoding {
-  static final Encoding UTF_8 = const Encoding("UTF-8");
-  static final Encoding ISO_8859_1 = const Encoding("ISO-8859-1");
-  static final Encoding ASCII = const Encoding("ASCII");
-  const Encoding(String this.name);
+  static final Encoding UTF_8 = const Encoding._internal("UTF-8");
+  static final Encoding ISO_8859_1 = const Encoding._internal("ISO-8859-1");
+  static final Encoding ASCII = const Encoding._internal("ASCII");
+  const Encoding._internal(String this.name);
   final String name;
 }