Fix static warning in lib/mirrors/method_mirror_source_test.

tests/lib/mirrors/method_mirror_source_test.dart

Index: tests/lib/mirrors/method_mirror_source_test.dart
diff --git a/tests/lib/mirrors/method_mirror_source_test.dart b/tests/lib/mirrors/method_mirror_source_test.dart
index 78834a33601b68ae6c05f475730e84445ccf0ce0..9b0945209d0c9a22c34a4224d26a8b27db0d2dc1 100644
--- a/tests/lib/mirrors/method_mirror_source_test.dart
+++ b/tests/lib/mirrors/method_mirror_source_test.dart
@@ -100,7 +100,8 @@ main() {
   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");
 }""");