add source maps support, fixes #50

[email protected]

Review URL: https://codereview.chromium.org/965033002

Author: John Messerly

pkg/dev_compiler/lib/devc.dart

@@ -118,7 +118,7 @@ CheckerResults _compileDart(
   if (outputDir != null) {
     var cg = options.outputDart
         ? new DartGenerator(outputDir, uri, rules, options)
-        : new JSGenerator(outputDir, uri, rules);
+        : new JSGenerator(outputDir, uri, rules, options);
     generators.add(cg);
   }