Don't crash when mapping stack chains.

test/source_map_stack_trace_test.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:source_maps/source_maps.dart';
 import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:source_map_stack_trace/source_map_stack_trace.dart';
 import 'package:test/test.dart';
 
@@ skipping 104 matching lines @@
                 new SourceLocation(12, line: 9, column: 1),
                 "\n" * 4));
 
     var mapping = parseJson(builder.build("foo.dart.js.map"));
     var frame = mapStackTrace(mapping, trace, sdkRoot: "sdk/").frames.first;
     expect(frame.uri, equals(Uri.parse("dart:async/foo.dart")));
     expect(frame.line, equals(2));
     expect(frame.column, equals(4));
   });
 
+  test("converts a stack chain", () {
+    var trace = new Chain([
+      new Trace.parse("foo.dart.js 10:11  foo"),
+      new Trace.parse("foo.dart.js 10:11  bar")
+    ]);
+    var traces = mapStackTrace(_simpleMapping, trace).traces;
+
+    var frame = traces.first.frames.single;
+    expect(frame.uri, equals(Uri.parse("foo.dart")));
+    expect(frame.member, equals("foo"));
+    expect(frame.line, equals(2));
+    expect(frame.column, equals(4));
+
+    frame = traces.last.frames.single;
+    expect(frame.uri, equals(Uri.parse("foo.dart")));
+    expect(frame.member, equals("bar"));
+    expect(frame.line, equals(2));
+    expect(frame.column, equals(4));
+  });
+
   group("cleans up", () {
     test("Firefox junk", () {
       expect(_prettify("foo/<"), equals("foo"));
       expect(_prettify("foo<"), equals("foo"));
     });
 
     test("arity indicators", () {
       expect(_prettify(r"foo$1"), equals("foo"));
       expect(_prettify(r"foo$1234"), equals("foo"));
     });
@@ skipping 33 matching lines @@
           equals("Foo.<fn>"));
     });
   });
 }
 
 /// Runs the mapper's prettification logic on [member] and returns the result.
 String _prettify(String member) {
   var trace = new Trace([new Frame(Uri.parse("foo.dart.js"), 10, 11, member)]);
   return mapStackTrace(_simpleMapping, trace).frames.first.member;
 }