Make source map paths relative

lib/src/compiler/compiler.dart

 // Copyright (c) 2016, 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 'dart:collection' show HashSet, Queue;
 import 'dart:convert' show JSON;
 import 'dart:io' show File;
 import 'package:analyzer/dart/element/element.dart' show LibraryElement;
 import 'package:analyzer/analyzer.dart'
     show AnalysisError, CompilationUnit, ErrorSeverity;
@@ skipping 345 matching lines @@
       printer = sourceMapContext;
     } else {
       printer = new JS.SimpleJavaScriptPrintingContext();
     }
 
     var tree = transformModuleFormat(format, moduleTree);
     tree.accept(
         new JS.Printer(opts, printer, localNamer: new JS.TemporaryNamer(tree)));
 
     if (options.sourceMap && options.sourceMapComment) {
-      printer.emit('\n//# sourceMappingURL=$mapUrl\n');
+      var relativeMapUrl = path
+          .toUri(path.relative(path.fromUri(mapUrl), from: path.dirname(jsUrl)))
+          .toString();
+      assert(path.dirname(jsUrl) == path.dirname(mapUrl));
+      printer.emit('\n//# sourceMappingURL=$relativeMapUrl\n');
     }
 
     Map builtMap;
     if (sourceMap != null) {
       builtMap = placeSourceMap(sourceMap.build(jsUrl), mapUrl);
     }
     return new JSModuleCode(printer.getText(), builtMap);
   }
 
   /// Similar to [getCode] but immediately writes the resulting files.
@@ skipping 29 matching lines @@
 
 /// Adjusts the source paths in [sourceMap] to be relative to [sourceMapPath],
 /// and returns the new map.
 // TODO(jmesserly): find a new home for this.
 Map placeSourceMap(Map sourceMap, String sourceMapPath) {
   var dir = path.dirname(sourceMapPath);
 
   var map = new Map.from(sourceMap);
   List list = new List.from(map['sources']);
   map['sources'] = list;
+  String relative(String uri) =>
+      path.toUri(path.relative(path.fromUri(uri), from: dir)).toString();
   for (int i = 0; i < list.length; i++) {
-    list[i] =
-        path.toUri(path.relative(path.fromUri(list[i]), from: dir)).toString();
+    list[i] = relative(list[i]);
   }
+  map['file'] = relative(map['file']);
   return map;
 }