Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/constant_expression_evaluate_test.dart

Index: tests/compiler/dart2js/constant_expression_evaluate_test.dart
diff --git a/tests/compiler/dart2js/constant_expression_evaluate_test.dart b/tests/compiler/dart2js/constant_expression_evaluate_test.dart
index 8ba312b480dbd6ee5ef4f3853bf08fa1d32edeba..a4a6b02c70c2eafb986e475e5b512b9bbeb62301 100644
--- a/tests/compiler/dart2js/constant_expression_evaluate_test.dart
+++ b/tests/compiler/dart2js/constant_expression_evaluate_test.dart
@@ -276,7 +276,7 @@ Future testData(TestData data) async {
   CompilationResult result = await runCompiler(
       memorySourceFiles: {'main.dart': source}, options: ['--analyze-all']);
   Compiler compiler = result.compiler;
-  var library = compiler.mainApp;
+  var library = compiler.frontendStrategy.elementEnvironment.mainLibrary;
   constants.forEach((String name, ConstantData data) {
     FieldElement field = library.localLookup(name);
     ConstantExpression constant = field.constant;