Support source maps in server mode

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 906251a38de95d6f5ee7fc419a12147f86a51f7b..a411957c9e51b597054c81dfdf3a415c5cd7dc35 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -3588,7 +3588,11 @@ class JSGenerator extends CodeGenerator {
         new JSCodegenVisitor(compiler, rules, library, _extensionTypes, fields);
     var module = codegen.emitLibrary(unit);
     var out = compiler.getOutputPath(library.source.uri);
-    return writeJsLibrary(module, out,
+    var flags = compiler.options;
+    var serverUri = flags.serverMode
+        ? Uri.parse('http://${flags.host}:${flags.port}/')
+        : null;
+    return writeJsLibrary(module, out, compiler.inputBaseDir, serverUri,
         emitSourceMaps: options.emitSourceMaps,
         arrowFnBindThisWorkaround: options.arrowFnBindThisWorkaround);
   }