Do not return the JSON as an escaped String to work around a strange v8 bug.

pkg/dev_compiler/web/stack_trace_mapper.dart

 // Copyright (c) 2017, 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.
 
 /// Standalone utility that manages loading source maps for all Dart scripts
 /// on the page compiled with DDC.
 ///
 /// Example JavaScript usage:
 /// $dartStackTraceUtility.addLoadedListener(function() {
 ///   // All Dart source maps are now loaded. It is now safe to start your
@@ skipping 21 matching lines @@
 import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 typedef void ReadyCallback();
 
 /// Global object DDC uses to see if a stack trace utility has been registered.
 @JS(r'$dartStackTraceUtility')
 external set dartStackTraceUtility(DartStackTraceUtility value);
 
 typedef String StackTraceMapper(String stackTrace);
-typedef String SourceMapProvider(String modulePath);
+typedef dynamic SourceMapProvider(String modulePath);
 typedef String SetSourceMapProvider(SourceMapProvider provider);
 
 @JS()
 @anonymous
 class DartStackTraceUtility {
   external factory DartStackTraceUtility(
       {StackTraceMapper mapper, SetSourceMapProvider setSourceMapProvider});
 }
 
+@JS('JSON.stringify')
+external String _stringify(dynamic json);
+
 /// Source mapping that is waits to parse source maps until they match the uri
 /// of a requested source map.
 ///
 /// This improves startup performance compared to using MappingBundle directly.
 /// The unparsed data for the source maps must still be loaded before
 /// LazyMapping is used.
 class LazyMapping extends Mapping {
   MappingBundle _bundle = new MappingBundle();
   SourceMapProvider _provider;
 
   LazyMapping(this._provider);
 
   List toJson() => _bundle.toJson();
 
   SourceMapSpan spanFor(int line, int column,
       {Map<String, SourceFile> files, String uri}) {
     if (uri == null) {
       throw new ArgumentError.notNull('uri');
     }
 
     if (!_bundle.containsMapping(uri)) {
       var rawMap = _provider(uri);
       if (rawMap != null) {
+        if (rawMap is! String) {
+          // The sourcemap was passed as regular JavaScript JSON.
+          rawMap = _stringify(rawMap);
+        }
         SingleMapping mapping = parse(rawMap);
         mapping
           ..targetUrl = uri
           ..sourceRoot = '${path.dirname(uri)}/';
         _bundle.addMapping(mapping);
       }
     }
     var span = _bundle.spanFor(line, column, files: files, uri: uri);
     // TODO(jacobr): we shouldn't have to filter out invalid sourceUrl entries
     // here.
@@ skipping 29 matching lines @@
 void setSourceMapProvider(SourceMapProvider provider) async {
   _mapping = new LazyMapping(provider);
 }
 
 main() {
   // Register with DDC.
   dartStackTraceUtility = new DartStackTraceUtility(
       mapper: allowInterop(mapper),
       setSourceMapProvider: allowInterop(setSourceMapProvider));
 }