Remove Compiler.mainApp and Compiler.mainFunction

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 4b28acb037314cc6bbea6f95c2850eee11a3288f..d19f1fb00f1cb6c7ba5451d7c59df16e252c08b0 100644
--- a/tests/compiler/dart2js/array_tracing_mirror_test.dart
+++ b/tests/compiler/dart2js/array_tracing_mirror_test.dart
@@ -34,7 +34,7 @@ main() {
   asyncTest(() async {
     var result = await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     var compiler = result.compiler;
-    var element = compiler.mainFunction;
+    var element = compiler.frontendStrategy.elementEnvironment.mainFunction;
     var code = compiler.backend.getGeneratedCode(element);
     Expect.isTrue(code.contains('return 2'), "Unexpected code:\n$code");
   });