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

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 2d4862cc96f5f81e32a411585a45be099a3a881d..c08dbde9a9385f3916cce9ae74f029558b4f9b88 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -268,7 +268,7 @@ class MockCompiler extends Compiler {
     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));