Remove legacy syntax support for library import

tests/language/library_prefixes.lib

Index: tests/language/library_prefixes.lib
===================================================================
--- tests/language/library_prefixes.lib	(revision 20447)
+++ tests/language/library_prefixes.lib	(working copy)
@@ -2,8 +2,8 @@
 // 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("LibraryPrefixes.lib");
+library LibraryPrefixes.lib;
 
-#import("library_prefixes_test1.lib");
-#import("library_prefixes_test2.lib", prefix:"other");
-#source("library_prefixes.dart");
+import "library_prefixes_test1.lib";
+import "library_prefixes_test2.lib" as other;
+part "library_prefixes.dart";