Fix libraryDepsVisitor to properly track prefixed library references

compiler/javatests/com/google/dart/compiler/end2end/inc/my.prefixed.app.dart

Index: compiler/javatests/com/google/dart/compiler/end2end/inc/my.prefixed.app.dart
diff --git a/compiler/javatests/com/google/dart/compiler/end2end/inc/my.prefixed.app.dart b/compiler/javatests/com/google/dart/compiler/end2end/inc/my.prefixed.app.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4efbc6c480d6582982f13ea029225194565cb18f
--- /dev/null
+++ b/compiler/javatests/com/google/dart/compiler/end2end/inc/my.prefixed.app.dart
@@ -0,0 +1,17 @@
+// 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.
+
+#import("some.prefixable.lib.dart", prefix: "prefix");
+
+class Prefix {
+  static final int foo = 43;
+
+  static getSource() {
+    return foo;
+  }
+
+  static getImport() {
+    return prefix.Prefix.foo;
+  }
+}