Fix attempt to find metadata of a library with no metadata.

tests/lib/mirrors/metadata_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.
 
 library test.metadata_test;
 
 import 'dart:mirrors';
 
 const string = 'a metadata string';
 
 const symbol = const Symbol('symbol');
 
 const hest = 'hest';
 
 @symbol @string
 class MyClass {
   @hest @hest @symbol
   var x;
   var y;
 
   @string @symbol @string
   myMethod() => 1;
+  myOtherMethod() => 2;
 }
 
 checkMetadata(DeclarationMirror mirror, List expectedMetadata) {
   List metadata = mirror.metadata.map((m) => m.reflectee).toList();
   if (metadata == null) {
     throw 'Null metadata on $mirror';
   }
   int expectedLength = expectedMetadata.length;
   int actualLength = metadata.length;
   if (expectedLength != actualLength) {
@@ skipping 17 matching lines @@
   }
 
   MirrorSystem mirrors = currentMirrorSystem();
   ClassMirror myClassMirror = reflectClass(MyClass);
   checkMetadata(myClassMirror, [symbol, string]);
   LibraryMirror lib = mirrors.findLibrary(const Symbol('test.metadata_test')).first;
   MethodMirror function = lib.functions[const Symbol('main')];
   checkMetadata(function, [symbol, string, symbol]);
   MethodMirror method = myClassMirror.methods[const Symbol('myMethod')];
   checkMetadata(method, [string, symbol, string]);
+  method = myClassMirror.methods[const Symbol('myOtherMethod')];
+  checkMetadata(method, []);
 
   VariableMirror xMirror = myClassMirror.variables[const Symbol('x')];
   checkMetadata(xMirror, [hest, hest, symbol]);
 
   VariableMirror yMirror = myClassMirror.variables[const Symbol('y')];
   checkMetadata(yMirror, []);
 
   // TODO(ahe): Test local functions.
 }