fix #27258, don't allow dynamic set of a final field

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

Index: pkg/dev_compiler/tool/input_sdk/private/js_mirrors.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_mirrors.dart b/pkg/dev_compiler/tool/input_sdk/private/js_mirrors.dart
index 9b2d9052f689a60ff329ddcf66ea46022e7b77ed..0a62b5d875121c7c4be21626b4b119011c357c54 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_mirrors.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_mirrors.dart
@@ -360,13 +360,7 @@ class JsClassMirror extends JsMirror implements ClassMirror {
       }
       var fields = _getFields(unwrapped);
       fields.forEach((symbol, t) {
-        var metadata = [];
-        if (t is List) {
-          metadata = t.skip(1).toList();
-          t = t[0];
-        }
-        _declarations[symbol] =
-            new JsVariableMirror._(symbol, _wrap(t), metadata);
+        _declarations[symbol] = new JsVariableMirror._fromField(symbol, t);
       });
       var methods = _getMethods(unwrapped);
       methods.forEach((symbol, ft) {
@@ -390,14 +384,7 @@ class JsClassMirror extends JsMirror implements ClassMirror {
       });
       var staticFields = _getStaticFields(unwrapped);
       staticFields.forEach((symbol, t) {
-        var name = getName(symbol);
-        var metadata = [];
-        if (t is List) {
-          metadata = t.skip(1).toList();
-          t = t[0];
-        }
-        _declarations[symbol] =
-            new JsVariableMirror._(symbol, _wrap(t), metadata);
+        _declarations[symbol] = new JsVariableMirror._fromField(symbol, t);
       });
       var statics = _getStatics(unwrapped);
       statics.forEach((symbol, ft) {
@@ -537,20 +524,26 @@ class JsVariableMirror extends JsMirror implements VariableMirror {
   final String _name;
   final TypeMirror type;
   final List<InstanceMirror> metadata;
+  final bool isFinal;
 
   // TODO(vsm): Refactor this out.
   Symbol get simpleName => _symbol;
 
   // TODO(vsm): Fix this
   final bool isStatic = false;
-  final bool isFinal = false;
 
-  JsVariableMirror._(Symbol symbol, Type t, List annotations)
+  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((a) => reflect(a)));
+        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'";
 }