Optimize getField by caching a closure generated from a specialized function kind.

runtime/lib/mirrors_impl.dart

Index: runtime/lib/mirrors_impl.dart
diff --git a/runtime/lib/mirrors_impl.dart b/runtime/lib/mirrors_impl.dart
index 310c9f7b48b997589ce658ab2ed42fdff2009365..cf5c54a781f1d0b5d8c68267463b5b3255da46f8 100644
--- a/runtime/lib/mirrors_impl.dart
+++ b/runtime/lib/mirrors_impl.dart
@@ -355,6 +355,33 @@ class _LocalInstanceMirror extends _LocalObjectMirror
     return reflect(this._invoke(_reflectee, _n(memberName), arguments, names));
   }
 
+  static Map _getFieldClosures = new Map.identity();
+  static Map _setFieldClosures = new Map.identity();
+
+  InstanceMirror getField(Symbol memberName) {
+    var f = _getFieldClosures[memberName];
+    if (f == null) {
+      f = _makeGetter(_n(memberName), _ScratchClass);
+      _getFieldClosures[memberName] = f;
+    }
+    return reflect(f(_reflectee));
+  }
+
+  InstanceMirror setField(Symbol memberName, value) {
+    var f = _setFieldClosures[memberName];
+    if (f == null) {
+      f = _makeSetter(_n(memberName), _ScratchClass);
+      _setFieldClosures[memberName] = f;
+    }
+    return reflect(f(_reflectee, value));
+  }
+
+  static _makeGetter(selector, scratchClass)
+      native "Mirrors_makeGetter";
+
+  static _makeSetter(selector, scratchClass)
+      native "Mirrors_makeSetter";
+
   _invoke(reflectee, functionName, arguments, argumentNames)
       native 'InstanceMirror_invoke';
 
@@ -368,6 +395,8 @@ class _LocalInstanceMirror extends _LocalObjectMirror
       native 'InstanceMirror_computeType';
 }
 
+class _ScratchClass {}
+
 class _LocalClosureMirror extends _LocalInstanceMirror
     implements ClosureMirror {
   _LocalClosureMirror(reflectee) : super(reflectee);