Frog: preserve initialization order.

frog/gen.dart

Index: frog/gen.dart
diff --git a/frog/gen.dart b/frog/gen.dart
index b5b2c7d6d70312c2ea7cfc6f296b220199665f28..f37a1912b116480f81ce7a70c766fab073e08eb5 100644
--- a/frog/gen.dart
+++ b/frog/gen.dart
@@ -1066,10 +1066,12 @@ class MethodGenerator implements TreeVisitor, CallingContext {
 
     if (method.isConstructor && newObject is ObjectValue) {
       var fields = newObject.dynamic.fields;
-      for (var field in fields.getKeys()) {
-        var value = fields[field];
-        if (value !== null) {
-          writer.writeln('this.${field.jsname} = ${value.code};');
+      for (var field in newObject.dynamic.fieldsInInitOrder) {
+        if (field !== null) {
+          var value = fields[field];
+          if (value !== null) {
+            writer.writeln('this.${field.jsname} = ${value.code};');
+          }
         }
       }
     }