Use shorter names for some standard properties on closures.

pkg/compiler/lib/src/js_backend/frequency_namer.dart

Index: pkg/compiler/lib/src/js_backend/frequency_namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/frequency_namer.dart b/pkg/compiler/lib/src/js_backend/frequency_namer.dart
index 9943b4d78e6acd3db34be132b05896e8176af3df..5f114d7e705176f998a041cacd70f66d6c565d63 100644
--- a/pkg/compiler/lib/src/js_backend/frequency_namer.dart
+++ b/pkg/compiler/lib/src/js_backend/frequency_namer.dart
@@ -21,6 +21,9 @@ class FrequencyBasedNamer extends Namer
   final String getterPrefix = 'g';
   final String setterPrefix = 's';
   final String callPrefix = ''; // this will create function names $<n>
+  String get requiredParameterField => r'$R';
+  String get defaultValuesField => r'$D';
+  String get operatorSignature => r'$S';
 
   jsAst.Name get staticsPropertyName =>
       _staticsPropertyName ??= getFreshName(instanceScope, 'static');