fixes #6, refactor to use AnalysisError/Listener throughout dev_compiler

test/report_test.dart

 // Copyright (c) 2015, 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.
 
 /// Tests for summary reporting.
 library dev_compiler.test.report_test;
 
 import 'package:test/test.dart';
 
 import 'package:dev_compiler/devc.dart';
@@ skipping 34 matching lines @@
         context: context, reporter: reporter).run();
 
     _verifySummary(GlobalSummary summary) {
       var mainLib = summary.loose['file:///main.dart'];
       expect(mainLib.messages.length, 2);
       var analyzerMsg = mainLib.messages[0];
       expect(analyzerMsg.kind, "AnalyzerMessage");
 
       var mainMessage = mainLib.messages[1];
       expect(mainMessage.kind, "StaticTypeError");
-      expect(mainMessage.level, "severe");
+      expect(mainMessage.level, "error");
       expect(mainMessage.span.text, '"hi"');
       expect(
           mainMessage.span.context, '  x = /*severe:StaticTypeError*/"hi";\n');
 
       var barLib = summary.packages['foo'].libraries['package:foo/bar.dart'];
       expect(barLib.messages.length, 1);
       var barMessage = barLib.messages[0];
       expect(barMessage.kind, "AssignmentCast");
       expect(barMessage.level, "info");
       expect(barMessage.span.text, 'x');
       expect(barMessage.span.context, '  int y = /*info:AssignmentCast*/x;\n');
     }
 
     var original = reporter.result;
     _verifySummary(original);
     _verifySummary(GlobalSummary.parse(original.toJsonMap()));
   });
 }