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

tests/compiler/dart2js_native/native_exception2_test.dart

Index: tests/compiler/dart2js_native/native_exception2_test.dart
diff --git a/tests/compiler/dart2js_native/native_exception2_test.dart b/tests/compiler/dart2js_native/native_exception2_test.dart
index cb432c4b7066c4397eb046e8d5b7a7c7508020a3..59cf5b0bd42fd83c491928bbf7b1dfb4feab3396 100644
--- a/tests/compiler/dart2js_native/native_exception2_test.dart
+++ b/tests/compiler/dart2js_native/native_exception2_test.dart
@@ -17,11 +17,14 @@ class NativeClass {
 
 makeNativeClass() native;
 
-setup() native """
-function NativeClass() {}
-makeNativeClass = function() { return new NativeClass; }
-self.nativeConstructor(NativeClass);
-""";
+setup() {
+  JS('', r"""
+(function(){
+  function NativeClass() {}
+  makeNativeClass = function() { return new NativeClass(); };
+  self.nativeConstructor(NativeClass);
+})()""");
+}
 
 main() {
   nativeTesting();