Make source map paths relative

lib/src/compiler/compiler.dart

Index: lib/src/compiler/compiler.dart
diff --git a/lib/src/compiler/compiler.dart b/lib/src/compiler/compiler.dart
index d9b6b9630283595e72a25147f74b18daa4671e59..e8b5a1697b3d16b7e2086ec0983e701b40156952 100644
--- a/lib/src/compiler/compiler.dart
+++ b/lib/src/compiler/compiler.dart
@@ -363,12 +363,14 @@ class JSModuleFile {
         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);
   }