Fix libraryDepsVisitor to properly track prefixed library references

compiler/javatests/com/google/dart/compiler/end2end/inc/some.prefixable.modified.lib.dart

Index: compiler/javatests/com/google/dart/compiler/end2end/inc/some.prefixable.modified.lib.dart
diff --git a/compiler/javatests/com/google/dart/compiler/end2end/inc/some.prefixable.modified.lib.dart b/compiler/javatests/com/google/dart/compiler/end2end/inc/some.prefixable.modified.lib.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6ffc6434544c9fb9d4a00ce8d512dfbec470914a
--- /dev/null
+++ b/compiler/javatests/com/google/dart/compiler/end2end/inc/some.prefixable.modified.lib.dart
@@ -0,0 +1,9 @@
+// Copyright (c) 2011, 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("some_lib");
+
+class Prefix {
+  static final int foo = 43;
+}