Fix static warning in lib/mirrors/method_mirror_source_test.

tests/lib/mirrors/method_mirror_source_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.
 
 import "dart:mirrors";
 import "package:expect/expect.dart";
 import "method_mirror_source_other.dart";
 
 expectSource(Mirror mirror, String source) {
   MethodMirror methodMirror;
@@ skipping 82 matching lines @@
   expectSource(reflect((){}), "(){}");
   expectSource(reflect((x,y,z) { return x*y*z; }), "(x,y,z) { return x*y*z; }");
   expectSource(reflect((e) => doSomething(e)), "(e) => doSomething(e)");
 
   namedClosure(x,y,z) => 1;
   var a = () {};
   expectSource(reflect(namedClosure), "namedClosure(x,y,z) => 1;");
   expectSource(reflect(a), "() {}");
 
   // Function at first line.
-  expectSource(reflectClass(SomethingInOther).owner.declarations[#main],
+  LibraryMirror otherLib = reflectClass(SomethingInOther).owner;
+  expectSource(otherLib.declarations[#main],
 """main() {
   print("Blah");
 }""");
 }