Add ImportElement and ExportElement

tests/compiler/dart2js/array_tracing_mirror_test.dart

Index: tests/compiler/dart2js/array_tracing_mirror_test.dart
diff --git a/tests/compiler/dart2js/array_tracing_mirror_test.dart b/tests/compiler/dart2js/array_tracing_mirror_test.dart
index 62a6de800b1cfdd2f9de7bc6f1193772606faaee..6a083ca18a62eeffae23b4d92dc6b364b692b639 100644
--- a/tests/compiler/dart2js/array_tracing_mirror_test.dart
+++ b/tests/compiler/dart2js/array_tracing_mirror_test.dart
@@ -34,9 +34,10 @@ main() {
   asyncTest(() async {
     var result = await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     var compiler = result.compiler;
-    var element = compiler.mainApp.findExported('main');
+    var element = compiler.mainFunction;
     var code = compiler.backend.getGeneratedCode(element);
-    Expect.isTrue(code.contains('return 2'));
+    Expect.isTrue(code.contains('return 2'),
+        "Unexpected code:\n$code");
   });
 }