Use messages for (some) public API errors

pkg/front_end/lib/kernel_generator.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Defines the front-end API for converting source code to Dart Kernel objects.
 library front_end.kernel_generator;
 
 import 'dart:async' show Future;
 import 'dart:async';
 
 import 'package:kernel/kernel.dart' show Program;
 
 import 'compiler_options.dart';
 import 'src/base/processed_options.dart';
+import 'src/fasta/fasta_codes.dart';
 import 'src/kernel_generator_impl.dart';
 
 /// Generates a kernel representation of the program whose main library is in
 /// the given [source].
 ///
 /// Intended for whole program (non-modular) compilation.
 ///
 /// Given the Uri of a file containing a program's `main` method, this function
 /// follows `import`, `export`, and `part` declarations to discover the whole
 /// program, and converts the result to Dart Kernel format.
 ///
 /// If `compileSdk` in [options] is true, the generated program will include
 /// code for the SDK.
 ///
 /// If summaries are provided in [options], the compiler will use them instead
 /// of compiling the libraries contained in those summaries. This is useful, for
 /// example, when compiling for platforms that already embed those sources (like
 /// the sdk in the standalone VM).
 ///
 /// The input [source] is expected to be a script with a main method, otherwise
 /// an error is reported.
 // TODO(sigmund): rename to kernelForScript?
 Future<Program> kernelForProgram(Uri source, CompilerOptions options) async {
   var pOptions = new ProcessedOptions(options, false, [source]);
   var program = (await generateKernel(pOptions))?.program;
   if (program == null) return null;
 
   if (program.mainMethod == null) {
-    pOptions.reportError("No 'main' method found.");
+    pOptions.reportMessage(messageMissingMain.withLocation(source, -1));
     return null;
   }
 
   return program;
 }
 
 /// Generates a kernel representation for a build unit containing [sources].
 ///
 /// A build unit is a collection of libraries that are compiled together.
 /// Libraries in the build unit may depend on each other and may have
@@ skipping 20 matching lines @@
 /// is not permitted to refer to a part file declared in another build unit).
 ///
 /// The return value is a [Program] object with no main method set. The
 /// [Program] includes external libraries for those libraries loaded through
 /// summaries.
 Future<Program> kernelForBuildUnit(
     List<Uri> sources, CompilerOptions options) async {
   return (await generateKernel(new ProcessedOptions(options, true, sources)))
       ?.program;
 }