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

tests/compiler/dart2js_native/native_field_invocation6_test.dart

Index: tests/compiler/dart2js_native/native_field_invocation6_test.dart
diff --git a/tests/compiler/dart2js_native/native_field_invocation6_test.dart b/tests/compiler/dart2js_native/native_field_invocation6_test.dart
index 95439c53b6f9a0dc8f3e865baa4a96ab128f36a5..17ed6606019a8752156d21190bffb67ff03c8a0b 100644
--- a/tests/compiler/dart2js_native/native_field_invocation6_test.dart
+++ b/tests/compiler/dart2js_native/native_field_invocation6_test.dart
@@ -7,13 +7,16 @@ import "native_testing.dart";
 makeA() native;
 nativeFirst(x, y) native;
 
-void setup() native """
-nativeFirst = function(x, y) { return x; }
-
-function A() {}
-makeA = function() { return new A; }
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  nativeFirst = function(x, y) { return x; };
+
+  function A() {}
+  makeA = function() { return new A() };
+  self.nativeConstructor(A);
+})()""");
+}
 
 @Native("A")
 class A {