Remove Compiler.mainApp and Compiler.mainFunction

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 541cf9e47d07a1f18428b35fa216df47e8fdbffe..d5034c908c2fc0209912edbd394e70eea80fbf60 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -969,8 +969,9 @@ class JavaScriptBackend {
       int mirrorCount =
           totalMethodCount - mirrorsCodegenAnalysis.preMirrorsMethodCount;
       double percentage = (mirrorCount / totalMethodCount) * 100;
-      DiagnosticMessage hint =
-          reporter.createMessage(compiler.mainApp, MessageKind.MIRROR_BLOAT, {
+      DiagnosticMessage hint = reporter.createMessage(
+          closedWorld.elementEnvironment.mainLibrary,
+          MessageKind.MIRROR_BLOAT, {
         'count': mirrorCount,
         'total': totalMethodCount,
         'percentage': percentage.round()