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

tests/compiler/dart2js_native/native_mixin_with_plain_test.dart

Index: tests/compiler/dart2js_native/native_mixin_with_plain_test.dart
diff --git a/tests/compiler/dart2js_native/native_mixin_with_plain_test.dart b/tests/compiler/dart2js_native/native_mixin_with_plain_test.dart
index 535c28dda948c5da5d6001ef522942f295e14763..aea5e6768428e55e76d33744d94a2901a91ca3d3 100644
--- a/tests/compiler/dart2js_native/native_mixin_with_plain_test.dart
+++ b/tests/compiler/dart2js_native/native_mixin_with_plain_test.dart
@@ -40,15 +40,18 @@ class D extends C with M {
 makeA() native;
 makeB() native;
 
-void setup() native """
-function A() {this.aa = 'aa'}
-function B() {this.aa = 'bb'}
-makeA = function(){return new A;};
-makeB = function(){return new B;};
-
-self.nativeConstructor(A);
-self.nativeConstructor(B);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {this.aa = 'aa'}
+  function B() {this.aa = 'bb'}
+  makeA = function(){return new A()};
+  makeB = function(){return new B()};
+
+  self.nativeConstructor(A);
+  self.nativeConstructor(B);
+})()""");
+}
 
 main() {
   nativeTesting();