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

tests/compiler/dart2js_native/native_named_constructors3_frog_test.dart

Index: tests/compiler/dart2js_native/native_named_constructors3_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_named_constructors3_frog_test.dart b/tests/compiler/dart2js_native/native_named_constructors3_frog_test.dart
index 668ba299b5e1fd4c5d313b15ba872526d62e1396..20debda492612c7bc53154acc6cdad5e2bb933fc 100644
--- a/tests/compiler/dart2js_native/native_named_constructors3_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_named_constructors3_frog_test.dart
@@ -23,13 +23,16 @@ class A {
 
 makeA(v) native;
 
-void setup() native """
-// This code is all inside 'setup' and so not accessible from the global scope.
-function A(arg) { this._x = arg; }
-A.prototype.foo = function(){ return this._x; }
-makeA = function(arg) { return new A(arg); }
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  // This code is inside 'setup' and so not accessible from the global scope.
+  function A(arg) { this._x = arg; }
+  A.prototype.foo = function(){ return this._x; };
+  makeA = function(arg) { return new A(arg); };
+  self.nativeConstructor(A);
+})()""");
+}
 
 main() {
   nativeTesting();