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

tests/compiler/dart2js_native/native_mixin_multiple_test.dart

Index: tests/compiler/dart2js_native/native_mixin_multiple_test.dart
diff --git a/tests/compiler/dart2js_native/native_mixin_multiple_test.dart b/tests/compiler/dart2js_native/native_mixin_multiple_test.dart
index 747e6cc5768616f0fcd6582cc726ccf5f251666f..2cbbb24a11a63b84dca628479fdf9a2952966753 100644
--- a/tests/compiler/dart2js_native/native_mixin_multiple_test.dart
+++ b/tests/compiler/dart2js_native/native_mixin_multiple_test.dart
@@ -29,15 +29,18 @@ class M2 {
 A makeA() native;
 B makeB() native;
 
-void setup() native """
-function A() {}
-function B() {}
-makeA = function(){return new A;};
-makeB = function(){return new B;};
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
+  function B() {}
+  makeA = function(){return new A()};
+  makeB = function(){return new B()};
 
-self.nativeConstructor(A);
-self.nativeConstructor(B);
-""";
+  self.nativeConstructor(A);
+  self.nativeConstructor(B);
+})()""");
+}
 
 main() {
   nativeTesting();