Use '--source-map' and '--out' options to support source maps from non-commandline.

sdk/lib/_internal/compiler/implementation/source_map_builder.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.
 
 library source_map_builder;
 
 
 import 'util/util.dart';
 import 'scanner/scannerlib.dart' show Token;
 import 'source_file.dart';
 import 'util/uri_extras.dart' show relativize;
 
 class SourceMapBuilder {
   static const int VLQ_BASE_SHIFT = 5;
   static const int VLQ_BASE_MASK = (1 << 5) - 1;
   static const int VLQ_CONTINUATION_BIT = 1 << 5;
   static const int VLQ_CONTINUATION_MASK = 1 << 5;
   static const String BASE64_DIGITS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmn'
                                       'opqrstuvwxyz0123456789+/';
 
   final Uri uri;
+  final Uri fileUri;
 
   List<SourceMapEntry> entries;
 
   Map<String, int> sourceUrlMap;
   List<String> sourceUrlList;
   Map<String, int> sourceNameMap;
   List<String> sourceNameList;
 
   int previousTargetLine;
   int previousTargetColumn;
   int previousSourceUrlIndex;
   int previousSourceLine;
   int previousSourceColumn;
   int previousSourceNameIndex;
   bool firstEntryInLine;
 
-  SourceMapBuilder(this.uri) {
+  SourceMapBuilder(this.uri, this.fileUri) {
     entries = new List<SourceMapEntry>();
 
     sourceUrlMap = new Map<String, int>();
     sourceUrlList = new List<String>();
     sourceNameMap = new Map<String, int>();
     sourceNameList = new List<String>();
 
     previousTargetLine = 0;
     previousTargetColumn = 0;
     previousSourceUrlIndex = 0;
@@ skipping 20 matching lines @@
     buffer.write(']');
   }
 
   String build(SourceFile targetFile) {
     StringBuffer mappingsBuffer = new StringBuffer();
     entries.forEach((SourceMapEntry entry) => writeEntry(entry, targetFile,
                                                          mappingsBuffer));
     StringBuffer buffer = new StringBuffer();
     buffer.write('{\n');
     buffer.write('  "version": 3,\n');
+    if (uri != null && fileUri != null) {
+      buffer.write('  "file": "${relativize(uri, fileUri, false)}",\n');
+    }
     buffer.write('  "sourceRoot": "",\n');
     buffer.write('  "sources": ');
     if (uri != null) {
       sourceUrlList =
           sourceUrlList.map((url) => relativize(uri, Uri.parse(url), false))
               .toList();
     }
     printStringListOn(sourceUrlList, buffer);
     buffer.write(',\n');
     buffer.write('  "names": ');
@@ skipping 124 matching lines @@
 }
 
 class OffsetSourceFileLocation extends SourceFileLocation {
   final int offset;
   final String sourceName;
   OffsetSourceFileLocation(SourceFile sourceFile, this.offset,
       [this.sourceName]) : super(sourceFile);
 
   String getSourceName() => sourceName;
 }