Use messages for (some) public API errors

pkg/front_end/test/kernel_generator_test.dart

 // Copyright (c) 2017, 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.
 
 import 'package:front_end/front_end.dart';
 import 'package:front_end/src/fasta/kernel/utils.dart';
 import 'package:front_end/src/testing/compiler_common.dart';
 import 'package:kernel/ast.dart';
 
 import 'package:test/test.dart';
@@ skipping 38 matching lines @@
 
       // Note: summaries created by the SDK today contain empty statements as
       // method bodies.
       expect(printMember.function.body is EmptyStatement, isTrue);
     });
 
     test('compiler requires a main method', () async {
       var errors = [];
       var options = new CompilerOptions()..onError = (e) => errors.add(e);
       await compileScript('a() => print("hi");', options: options);
-      expect('${errors.first}', contains("No 'main' method found"));
+      expect(errors.first.message, contains("No 'main' method found"));

[Paul Berry, 2017/07/11 16:00:59]

Would it be possible to change these test assertions to refer to the generated
code?  If we did that, then later if we changed the error message text, we would
not have to update the test.

[Siggi Cherem (dart-lang), 2017/07/12 00:29:46]

In the future, yes, right now we don't expose the code in the public API. Added
TODO

     });
 
     test('default error handler throws', () async {
       var exceptionThrown = false;
       try {
         await compileScript('a() => print("hi");');
-      } catch (e) {
+      } on CompilationError catch (e) {
         exceptionThrown = true;
-        expect('$e', contains("No 'main' method found"));
+        expect(e.message, contains("No 'main' method found"));
       }
       expect(exceptionThrown, isTrue);
     });
 
     test('generated program contains source-info', () async {
       var program = await compileScript('a() => print("hi"); main() {}',
           fileName: 'a.dart');
       // Kernel always store an empty '' key in the map, so there is always at
       // least one. Having more means that source-info is added.
       expect(program.uriToSource.keys.length, greaterThan(1));
@@ skipping 60 matching lines @@
     });
 
     test('compiler by default is hermetic', () async {
       var errors = [];
       var options = new CompilerOptions()..onError = (e) => errors.add(e);
       var sources = {
         'a.dart': 'import "b.dart"; a() => print("hi");',
         'b.dart': ''
       };
       await compileUnit(['a.dart'], sources, options: options);
-      expect(errors.first.toString(), contains('Invalid access'));
+      expect(errors.first.message, contains('Invalid access'));
       errors.clear();
 
       await compileUnit(['a.dart', 'b.dart'], sources, options: options);
       expect(errors, isEmpty);
     });
 
     test('chaseDependencies=true removes hermetic restriction', () async {
       var errors = [];
       var options = new CompilerOptions()
         ..chaseDependencies = true
@@ skipping 41 matching lines @@
       checkDCallsC(unitD1);
 
       var unitD2 = await compileUnit(['d.dart'], sources,
           inputSummaries: ['bc.dill', 'a.dill']);
       checkDCallsC(unitD2);
     });
 
     // TODO(sigmund): add tests with trimming dependencies
   });
 }