Support new library/import syntax

tests/language/prefix_new_test1.dart

+// Copyright (c) 2012, 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 PrefixTest1;
+
+import "prefix_test2.dart" as test2;
+
+class Prefix {
+  static const int foo = 43;
+
+  static getSource() {
+    return foo;
+  }
+
+  static getImport() {
+    return test2.Prefix.foo;
+  }
+}