dart2js_native tests: Migrate 'native STRING' to JS-calls

tests/compiler/dart2js_native/field_type_test.dart

Index: tests/compiler/dart2js_native/field_type_test.dart
diff --git a/tests/compiler/dart2js_native/field_type_test.dart b/tests/compiler/dart2js_native/field_type_test.dart
index 7f844e8cd10f2183e63e7a00eab80fb0507daee6..8e2e83d42f5aa31995ec21a998f7edbbe99a8dba 100644
--- a/tests/compiler/dart2js_native/field_type_test.dart
+++ b/tests/compiler/dart2js_native/field_type_test.dart
@@ -43,12 +43,15 @@ class ModelSource {
   toString() => 'ModelSource($name)';
 }
 
-void setup() native """
-// This code is all inside 'setup' and so not accessible from the global scope.
-function Node(parent){ this.parentNode = parent; }
-makeNode = function(p){return new Node(p);};
-self.nativeConstructor(Node);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  // This code is inside 'setup' and so not accessible from the global scope.
+  function Node(parent){ this.parentNode = parent; }
+  makeNode = function(p){return new Node(p);};
+  self.nativeConstructor(Node);
+})()""");
+}
 
 main() {
   nativeTesting();