Use messages for (some) public API errors

pkg/front_end/test/src/base/processed_options_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 'dart:async';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/base/processed_options.dart';
 import 'package:front_end/src/fasta/fasta.dart' show ByteSink;
@@ skipping 144 matching lines @@
 
   test_validateOptions_root_doesnt_exists() async {
     var sdkRoot = Uri.parse('file:///sdk/root');
     var errors = [];
     var raw = new CompilerOptions()
       ..sdkRoot = sdkRoot
       ..fileSystem = fileSystem
       ..onError = (e) => errors.add(e);
     var options = new ProcessedOptions(raw);
     expect(await options.validateOptions(), isFalse);
-    expect('${errors.first}', contains("SDK root directory not found"));
+    expect(errors.first.message, contains("SDK root directory not found"));
   }
 
   test_validateOptions_summary_exists() async {
     var sdkSummary = Uri.parse('file:///sdk/root/outline.dill');
     fileSystem.entityForUri(sdkSummary).writeAsStringSync('\n');
 
     var errors = [];
     var raw = new CompilerOptions()
       ..sdkSummary = sdkSummary
       ..fileSystem = fileSystem
       ..onError = (e) => errors.add(e);
     var options = new ProcessedOptions(raw);
     var result = await options.validateOptions();
     expect(errors, isEmpty);
     expect(result, isTrue);
   }
 
   test_validateOptions_summary_doesnt_exists() async {
     var sdkSummary = Uri.parse('file:///sdk/root/outline.dill');
     var errors = [];
     var raw = new CompilerOptions()
       ..sdkSummary = sdkSummary
       ..fileSystem = fileSystem
       ..onError = (e) => errors.add(e);
     var options = new ProcessedOptions(raw);
     expect(await options.validateOptions(), isFalse);
-    expect('${errors.first}', contains("SDK summary not found"));
+    expect(errors.first.message, contains("SDK summary not found"));
   }
 
   test_validateOptions_inferred_summary_exists() async {
     var sdkRoot = Uri.parse('file:///sdk/root/');
     var sdkSummary = Uri.parse('file:///sdk/root/outline.dill');
     fileSystem.entityForUri(sdkRoot).writeAsStringSync('\n');
     fileSystem.entityForUri(sdkSummary).writeAsStringSync('\n');
 
     var errors = [];
     var raw = new CompilerOptions()
@@ skipping 10 matching lines @@
     var sdkRoot = Uri.parse('file:///sdk/root/');
     var sdkSummary = Uri.parse('file:///sdk/root/outline.dill');
     fileSystem.entityForUri(sdkRoot).writeAsStringSync('\n');
     var errors = [];
     var raw = new CompilerOptions()
       ..sdkSummary = sdkSummary
       ..fileSystem = fileSystem
       ..onError = (e) => errors.add(e);
     var options = new ProcessedOptions(raw);
     expect(await options.validateOptions(), isFalse);
-    expect('${errors.first}', contains("SDK summary not found"));
+    expect(errors.first.message, contains("SDK summary not found"));
   }
 }