Don't assign to final 'trace' parameter in catch clause.

lib/src/barback/barback_server.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 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
 import "package:crypto/crypto.dart";
@@ skipping 117 matching lines @@
         // implicitly support that too. This follows Apache's behavior.
         logRequest(request, "302 Redirect to /${request.url}/");
         return new shelf.Response.found('/${request.url}/');
       }).catchError((newError, newTrace) {
         // If we find neither the original file or the index, we should report
         // the error about the original to the user.
         throw newError is AssetNotFoundException ? error : newError;
       });
     }).catchError((error, trace) {
       if (error is! AssetNotFoundException) {
-        trace = new Chain.forTrace(trace);
-        logRequest(request, "$error\n$trace");
+        var chain = new Chain.forTrace(trace);
+        logRequest(request, "$error\n$chain");
 
-        addError(error, trace);
+        addError(error, chain);
         close();
         return new shelf.Response.internalServerError();
       }
 
       addResult(new BarbackServerResult._failure(request.url, id, error));
       return notFound(request, asset: id);
     }).then((response) {
       // Allow requests of any origin to access "pub serve". This is useful for
       // running "pub serve" in parallel with another development server. Since
       // "pub serve" is only used as a development server and doesn't require
@@ skipping 38 matching lines @@
     } catch (error, trace) {
       addResult(new BarbackServerResult._failure(request.url, asset.id, error));
 
       // If we couldn't read the asset, handle the error gracefully.
       if (error is FileSystemException) {
         // Assume this means the asset was a file-backed source asset
         // and we couldn't read it, so treat it like a missing asset.
         return notFound(request, error: error.toString(), asset: asset.id);
       }
 
-      trace = new Chain.forTrace(trace);
-      logRequest(request, "$error\n$trace");
+      var chain = new Chain.forTrace(trace);
+      logRequest(request, "$error\n$chain");
 
       // Otherwise, it's some internal error.
       return new shelf.Response.internalServerError(body: error.toString());
     }
   }
 }
 
 /// The result of the server handling a URL.
 ///
 /// Only requests for which an asset was requested from barback will emit a
@@ skipping 23 matching lines @@
       : error = null,
         isCached = false;
 
   BarbackServerResult._cached(this.url, this.id)
       : error = null,
         isCached = true;
 
   BarbackServerResult._failure(this.url, this.id, this.error)
       : isCached = false;
 }