Only support the new source_span spans in barback.

sdk/lib/_internal/pub/lib/src/log.dart

 // Copyright (c) 2012, 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.
 
 /// Message logging.
 library pub.log;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
-import 'package:source_maps/source_maps.dart';
 import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import 'exceptions.dart';
 import 'io.dart';
 import 'progress.dart';
 import 'transcript.dart';
 import 'utils.dart';
 
 /// The singleton instance so that we can have a nice api like:
@@ skipping 285 matching lines @@
 }
 
 /// Logs an exception.
 void exception(exception, [StackTrace trace]) {
   if (exception is SilentException) return;
 
   var chain = trace == null ? new Chain.current() : new Chain.forTrace(trace);
 
   // This is basically the top-level exception handler so that we don't
   // spew a stack trace on our users.
-  if (exception is SpanException) {
-    error(exception.toString(useColors: canUseSpecialChars));
-  } else if (exception is SourceSpanException) {
+  if (exception is SourceSpanException) {
     error(exception.toString(color: canUseSpecialChars));
   } else {
     error(getErrorMessage(exception));
   }
   fine("Exception type: ${exception.runtimeType}");
 
   if (json.enabled) {
     if (exception is UsageException) {
       // Don't print usage info in JSON output.
       json.error(exception.message);
@@ skipping 171 matching lines @@
   /// Always prints to stdout.
   void error(error, [stackTrace]) {
     var errorJson = {"error": error.toString()};
 
     if (stackTrace == null && error is Error) stackTrace = error.stackTrace;
     if (stackTrace != null) {
       errorJson["stackTrace"] = new Chain.forTrace(stackTrace).toString();
     }
 
     // If the error came from a file, include the path.
-    if ((error is SpanException || error is SourceSpanException) &&
-        error.span.sourceUrl != null) {
+    if (error is SourceSpanException && error.span.sourceUrl != null) {
       errorJson["path"] = p.fromUri(error.span.sourceUrl);
     }
 
     if (error is FileException) {
       errorJson["path"] = error.path;
     }
 
     this.message(errorJson);
   }
 
   /// Encodes [message] to JSON and prints it if JSON output is enabled.
   void message(message) {
     if (!enabled) return;
 
     print(JSON.encode(message));
   }
 }