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');
+      // Assume these are in the same directory and use relative paths.

[Jennifer Messerly, 2016/08/26 23:06:37]

I don't understand why we need this assumption. Can't you just make the URIs
relative to each other?

see placeSourceMap's `path.toUri(path.relative(path.fromUri(list[i]), from:
dir)).toString()` code, is it not working?

[vsm, 2016/08/26 23:19:48]

Done.

+      assert(path.dirname(jsUrl) == path.dirname(mapUrl));
+      printer.emit('\n//# sourceMappingURL=${path.basename(mapUrl)}\n');
     }
 
     Map builtMap;
     if (sourceMap != null) {
-      builtMap = placeSourceMap(sourceMap.build(jsUrl), mapUrl);
+      builtMap = placeSourceMap(sourceMap.build(path.basename(jsUrl)), mapUrl);

[Jennifer Messerly, 2016/08/26 23:06:37]

here too, I don't see why this is needed, the whole point of placeSourceMap is
to do this relative path adjustment.

[vsm, 2016/08/26 23:19:48]

done - see below.

     }
     return new JSModuleCode(printer.getText(), builtMap);
   }
 
   /// Similar to [getCode] but immediately writes the resulting files.
   ///
   /// If [mapPath] is not supplied but [options.sourceMap] is set, mapPath
   /// will default to [jsPath].map.
   void writeCodeSync(ModuleFormat format, String jsPath, [String mapPath]) {
     if (mapPath == null) mapPath = jsPath + '.map';
@@ skipping 30 matching lines @@
 
   var map = new Map.from(sourceMap);
   List list = new List.from(map['sources']);
   map['sources'] = list;
   for (int i = 0; i < list.length; i++) {
     list[i] =
         path.toUri(path.relative(path.fromUri(list[i]), from: dir)).toString();
   }
   return map;
 }