fix #29530, field metadata generation

pkg/dev_compiler/tool/input_sdk/private/js_mirrors.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart._js_mirrors;
 
 import 'dart:mirrors';
 import 'dart:_foreign_helper' show JS;
 import 'dart:_internal' as _internal;
 
@@ skipping 523 matching lines @@
   Symbol get simpleName => _symbol;
 
   // TODO(vsm): Fix this
   final bool isStatic = false;
 
   JsVariableMirror._(Symbol symbol, Type t, List annotations,
       {this.isFinal: false})
       : _symbol = symbol,
         _name = getName(symbol),
         type = reflectType(t),
-        metadata =
-            new List<InstanceMirror>.unmodifiable(annotations.map(reflect));
+        metadata = new List<InstanceMirror>.unmodifiable(
+            annotations?.map(reflect) ?? []);
 
   JsVariableMirror._fromField(Symbol symbol, fieldInfo)
       : this._(symbol, _wrap(JS('', '#.type', fieldInfo)),
             JS('', '#.metadata', fieldInfo),
             isFinal: JS('bool', '#.isFinal', fieldInfo));
 
   String toString() => "VariableMirror on '$_name'";
 }
 
 class JsParameterMirror extends JsVariableMirror implements ParameterMirror {
@@ skipping 91 matching lines @@
       var param =
           new JsParameterMirror._(new Symbol(''), _wrap(type), metadata);
       params[i + args.length] = param;
     }
 
     _params = new List.unmodifiable(params);
   }
 
   String toString() => "MethodMirror on '$_name'";
 }