Clean up error handling.

dart/tests/compiler/dart2js/diagnose_ambiguous_test.dart

 // Copyright (c) 2013, 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:expect/expect.dart';
 import 'memory_source_file_helper.dart';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart'
        show Diagnostic;
 
@@ skipping 30 matching lines @@
       'memory:library.dart:30:34:Info: "function(fisk)" is (re)exported by '
       'multiple libraries.:info',
       'memory:library.dart:41:45:Info: "function(hest)" is defined here.'
       ':info',
       'memory:main.dart:0:22:Info: "class(Fisk)" is imported here.:info',
       'memory:main.dart:0:22:Info: "function(fisk)" is imported here.:info',
       'memory:main.dart:0:22:Info: "function(hest)" is imported here.:info',
       'memory:main.dart:23:46:Info: "class(Fisk)" is imported here.:info',
       'memory:main.dart:23:46:Info: "function(fisk)" is imported here.:info',
       'memory:main.dart:23:46:Info: "function(hest)" is imported here.:info',
-      'memory:main.dart:59:63:Warning: duplicate import of Fisk:warning',
-      'memory:main.dart:76:80:duplicate import of fisk:error',
-      'memory:main.dart:86:90:duplicate import of hest:error'
+      'memory:main.dart:59:63:Warning: Duplicate import of "Fisk".:warning',
+      'memory:main.dart:76:80:Error: Duplicate import of "fisk".:error',
+      'memory:main.dart:86:90:Error: Duplicate import of "hest".:error'
   ];
   Expect.listEquals(expected, diagnostics);
   Expect.isTrue(compiler.compilationFailed);
 }
 
 const Map MEMORY_SOURCE_FILES = const {
   'main.dart': """
 import 'library.dart';
 import 'exporter.dart';
 
@@ skipping 14 matching lines @@
 hest() {}
 """,
   'exporter.dart': """
 library exporter;
 
 export 'library.dart';
 
 hest() {}
 """,
 };