Fix type error.

dart/tests/compiler/dart2js_extra/mirror_printer_test.dart

 // Copyright (c) 2013, 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.
 
 /// Prints all information about all mirrors. This tests that it is possible to
 /// enumerate all reflective information without crashing.
 
 // Note: Adding imports below is fine for regression tests. For example,
 // 'crash_library_metadata.dart' is imported to ensure the compiler doesn't
 // crash.
@@ skipping 56 matching lines @@
       if (first) {
         first = false;
       } else {
         buffer.write(', ');
       }
       buffer
           ..write(n(name))
           ..write(': ')
           ..write(stringifyInstance(mirror.getField(name)));
     });
-    return buffer
-        ..write(')')
-        ..toString();
+    buffer.write(')');
+    return buffer.toString();
   }
 
   String stringifyMetadata(InstanceMirror mirror) {
     return '@${stringifyInstance(mirror)}';
   }
 
   bool writeType(TypeMirror mirror) {
     if (mirror == null || mirror == dynamicType) return false;
     w('${n(mirror.simpleName)} ');
     return true;
@@ skipping 86 matching lines @@
     return buffer;
   }
 }
 
 main() {
   print(MirrorPrinter.stringify(currentMirrorSystem().libraries));
   // Clear the nodes to avoid confusing the fine test framework (by "fine" I
   // mean something else) -- ahe.
   document.body.nodes.clear();
 }