Split loader from the rest of the compiler. This adds several abstractions to

pkg/compiler/lib/src/patch_parser.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.
 
 /**
  * This library contains the infrastructure to parse and integrate patch files.
  *
  * Three types of elements can be patched: [LibraryElement], [ClassElement],
  * [FunctionElement]. Patches are introduced in patch libraries which are loaded
  * together with the corresponding origin library. Which libraries that are
@@ skipping 154 matching lines @@
       : this._enableConditionalDirectives = enableConditionalDirectives,
         super(compiler);
 
   /**
    * Scans a library patch file, applies the method patches and
    * injections to the library, and returns a list of class
    * patches.
    */
   Future patchLibrary(LibraryLoader loader,
                       Uri patchUri, LibraryElement originLibrary) {
-    return compiler.readScript(originLibrary, patchUri)
+    return compiler.readScript(patchUri, originLibrary)
         .then((Script script) {
       var patchLibrary = new LibraryElementX(script, null, originLibrary);
       return reporter.withCurrentElement(patchLibrary, () {
         loader.registerNewLibrary(patchLibrary);
         reporter.withCurrentElement(patchLibrary.entryCompilationUnit, () {
           // This patches the elements of the patch library into [library].
           // Injected elements are added directly under the compilation unit.
           // Patch elements are stored on the patched functions or classes.
           scanLibraryElements(patchLibrary.entryCompilationUnit);
         });
@@ skipping 510 matching lines @@
 
 class PatchVersion {
   final String tag;
 
   const PatchVersion(this.tag);
 
   bool isActive(String patchTag) => tag == null || tag == patchTag;
 
   String toString() => 'PatchVersion($tag)';
 }