Implement ParameterMirror.metadata.

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 4d211ec92db990fba528430ce4ae3ce70a20e5a6..1f413870963b0b36d8be9f93c6698b4b12388b50 100644
--- a/runtime/lib/mirrors_impl.dart
+++ b/runtime/lib/mirrors_impl.dart
@@ -1091,7 +1091,8 @@ class _LocalParameterMirrorImpl extends _LocalVariableMirrorImpl
                             this.isOptional,
                             this.isNamed,
                             bool isFinal,
-                            this._defaultValueReflectee)
+                            this._defaultValueReflectee,
+                            this._unmirroredMetadata)
       : super(reflectee,
               simpleName,
               owner,
@@ -1102,6 +1103,7 @@ class _LocalParameterMirrorImpl extends _LocalVariableMirrorImpl
   final int _position;
   final bool isOptional;
   final bool isNamed;
+  final List _unmirroredMetadata;
 
   Object _defaultValueReflectee;
   InstanceMirror _defaultValue;
@@ -1117,10 +1119,9 @@ class _LocalParameterMirrorImpl extends _LocalVariableMirrorImpl
 
   bool get hasDefaultValue => _defaultValueReflectee != null;
 
-  // TODO(11418): Implement.
   List<InstanceMirror> get metadata {
-    throw new UnimplementedError(
-        'ParameterMirror.metadata is not implemented');
+    if ( _unmirroredMetadata == null) return const [];
+    return _unmirroredMetadata.map(reflect).toList(growable:false);
   }
 
   TypeMirror _type = null;