Fix hints reported by analyzer.

pkg/compiler/lib/src/helpers/trace.dart

 // Copyright (c) 2014, 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 '../common.dart';
 import '../util/util.dart';
 
 /// Function signature for [trace].
 typedef void Trace(String message,
     {bool condition(String stackTrace), int limit, bool throwOnPrint});
@@ skipping 84 matching lines @@
   int rangeStart = offset;
   int rangeEnd = limit == null ? null : rangeStart + limit;
   try {
     throw '';
   } catch (_, stackTrace) {
     return new StackTraceLines.fromTrace(stackTrace,
         rangeStart: offset,
         rangeEnd: rangeEnd,
         filePrefix: stackTraceFilePrefix);
   }
-  return null;
 }
 
 /// A stack trace as a sequence of [StackTraceLine]s.
 class StackTraceLines {
   final List<StackTraceLine> lines;
   final int maxFileLength;
   final int maxLineNoLength;
   final int maxColumnNoLength;
 
   factory StackTraceLines.fromTrace(StackTrace s,
@@ skipping 223 matching lines @@
   }
   for (int index = text.length; index < intendedLength; index++) {
     int dotsIndex = index % dotsLength;
     sb.write(dots.substring(dotsIndex, dotsIndex + 1));
   }
   if (padLeft) {
     sb.write(text);
   }
   return sb.toString();
 }