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

tests/compiler/dart2js_native/native_mixin_multiple2_test.dart

Index: tests/compiler/dart2js_native/native_mixin_multiple2_test.dart
diff --git a/tests/compiler/dart2js_native/native_mixin_multiple2_test.dart b/tests/compiler/dart2js_native/native_mixin_multiple2_test.dart
index 992a40e629d5dbb6ae6e892a1f9ff3f50b78e4fe..0747e6b47f87ad28d155386eaa6583bc8087e16f 100644
--- a/tests/compiler/dart2js_native/native_mixin_multiple2_test.dart
+++ b/tests/compiler/dart2js_native/native_mixin_multiple2_test.dart
@@ -26,12 +26,15 @@ class M3 {}
 
 makeB() native;
 
-void setup() native """
-function B() {}
-makeB = function(){return new B;};
-
-self.nativeConstructor(B);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function B() {}
+  makeB = function(){return new B()};
+
+  self.nativeConstructor(B);
+})()""");
+}
 
 main() {
   nativeTesting();