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

tests/compiler/dart2js/mock_compiler.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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 250 matching lines @@
                            Uri resolvedUri, Spannable spannable) => resolvedUri;
 
   // The mock library doesn't need any patches.
   Uri resolvePatchUri(String dartLibraryName) {
     if (dartLibraryName == 'core') {
       return PATCH_CORE;
     }
     return null;
   }
 
-  Future<Script> readScript(Spannable node, Uri uri) {
+  Future<Script> readScript(Uri uri, [Spannable spannable]) {
     SourceFile sourceFile = sourceFiles[uri.toString()];
     if (sourceFile == null) throw new ArgumentError(uri);
     return new Future.value(new Script(uri, uri, sourceFile));
   }
 
   Element lookupElementIn(ScopeContainerElement container, name) {
     Element element = container.localLookup(name);
     return element != null
         ? element
         : new ErroneousElementX(null, null, name, container);
@@ skipping 89 matching lines @@
       trustTypeAnnotations: trustTypeAnnotations,
       enableTypeAssertions: enableTypeAssertions,
       enableUserAssertions: enableUserAssertions,
       expectedErrors: expectedErrors,
       expectedWarnings: expectedWarnings,
       outputProvider: outputProvider);
   compiler.registerSource(uri, code);
   compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler;
 }