Fix bugs in the log viewer

pkg/analysis_server/tool/instrumentation/log/log.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.
 
 /**
  * A representation of the contents of an instrumentation log.
  */
 library analysis_server.tool.instrumentation.log.log;
 
 import 'dart:convert';
+import 'dart:math' as math;
 
 import 'package:analyzer/instrumentation/instrumentation.dart';
 
 /**
  * A boolean-valued function of one argument.
  */
 typedef bool Predicate<T>(T value);
 
 /**
  * A description of a group of log entries.
@@ skipping 276 matching lines @@
       return buffer.toString();
     }
 
     List<String> extraLines = <String>[];
     for (int i = logContent.length - 1; i >= 0; i--) {
       String line = logContent[i];
       if (isStartOfEntry(line)) {
         if (extraLines.isNotEmpty) {
           logContent[i] = merge(line, extraLines);
         }
+        extraLines.clear();
       } else {
         logContent.removeAt(i);
         extraLines.insert(0, line);
       }
     }
     if (extraLines.isNotEmpty) {
-      throw new StateError(
-          '${extraLines.length} non-entry lines before any entry');
+      int count = math.min(extraLines.length, 10);
+      StringBuffer buffer = new StringBuffer();
+      buffer.writeln('${extraLines.length} non-entry lines before any entry');
+      buffer.writeln('First $count lines:');
+      for (int i = 0; i < count; i++) {
+        buffer.writeln(extraLines[i]);
+      }
+      throw new StateError(buffer.toString());
     }
   }
 
   /**
    * Parse the given [logContent] into a list of log entries.
    */
   void _parseLogContent(List<String> logContent) {
     if (_isSessionData(logContent)) {
       if (logContent[0].startsWith('-----')) {
         logContent.removeAt(0);
@@ skipping 597 matching lines @@
     int slash = context.lastIndexOf('/');
     if (slash < 0) {
       slash = context.lastIndexOf('\\');
     }
     if (slash >= 0) {
       String prefix = context.substring(0, slash);
       _target = _target.replaceAll(prefix, '...');
     }
   }
 }