Don't crash when mapping stack chains.

lib/source_map_stack_trace.dart

Index: lib/source_map_stack_trace.dart
diff --git a/lib/source_map_stack_trace.dart b/lib/source_map_stack_trace.dart
index 9c65dd9a9a0055f4dbcb0959e97d9c3c23b6d2a1..a5a22b31727c9ede2ff51b1054f05159153f1586 100644
--- a/lib/source_map_stack_trace.dart
+++ b/lib/source_map_stack_trace.dart
@@ -25,8 +25,8 @@ StackTrace mapStackTrace(Mapping sourceMap, StackTrace stackTrace,
     {bool minified: false, packageRoot, sdkRoot}) {
   if (stackTrace is Chain) {
     return new Chain(stackTrace.traces.map((trace) {
-      return mapStackTrace(trace, sourceMap,
-          minified: minified, packageRoot: packageRoot, sdkRoot: sdkRoot);
+      return new Trace.from(mapStackTrace(sourceMap, trace,
+          minified: minified, packageRoot: packageRoot, sdkRoot: sdkRoot));
     }));
   }