Fix a crashing bug.

lib/source_map_stack_trace.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.
 
 import 'package:package_resolver/package_resolver.dart';
 import 'package:path/path.dart' as p;
 import 'package:source_maps/source_maps.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 /// Convert [stackTrace], a stack trace generated by dart2js-compiled
@@ skipping 58 matching lines @@
     if (span == null) return null;
 
     var sourceUrl = span.sourceUrl.toString();
     if (sdkRoot != null && p.url.isWithin(sdkLib, sourceUrl)) {
       sourceUrl = "dart:" + p.url.relative(sourceUrl, from: sdkLib);
     } else if (packageResolver != null) {
       if (packageResolver.packageRoot != null &&
           p.url.isWithin(packageResolver.packageRoot.toString(), sourceUrl)) {
         sourceUrl = "package:" + p.url.relative(sourceUrl,
             from: packageResolver.packageRoot.toString());
-      } else {
+      } else if (packageResolver.packageConfig != null) {
         for (var package in packageResolver.packageConfigMap.keys) {
           var packageUrl = packageResolver.packageConfigMap[package].toString();
           if (!p.url.isWithin(packageUrl, sourceUrl)) continue;
 
           sourceUrl = "package:$package/" +
               p.url.relative(sourceUrl, from: packageUrl);
           break;
         }
       }
     }
@@ skipping 31 matching lines @@
       .replaceAll(new RegExp(r"[a-zA-Z_0-9]+\$"), "")
       // Get rid of the static method prefix. The class name also exists in the
       // invocation, so we're not getting rid of any information.
       .replaceAll(new RegExp(r"^[a-zA-Z_0-9]+.(static|dart)."), "")
       // Convert underscores after identifiers to dots. This runs the risk of
       // incorrectly converting members that contain underscores, but those are
       // contrary to the style guide anyway.
       .replaceAllMapped(new RegExp(r"([a-zA-Z0-9]+)_"),
           (match) => match[1] + ".");
 }