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

pkg/compiler/lib/src/script.dart

 // Copyright (c) 2011, 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 dart2js.script;
 
 import 'io/source_file.dart';
 
 class Script {
   final SourceFile file;
 
   /**
    * The readable URI from which this script was loaded.
    *
    * See [LibraryLoader] for terminology on URIs.
    */
   final Uri readableUri;
 
 
   /**
    * The resource URI from which this script was loaded.
    *
    * See [LibraryLoader] for terminology on URIs.
    */
   final Uri resourceUri;
 
   /// This script was synthesized.
   final bool isSynthesized;
 
-  Script(
-      this.readableUri, this.resourceUri, this.file,
-      {this.isSynthesized: false});
+  Script(this.readableUri, this.resourceUri, this.file)
+      : isSynthesized = false;
+
+  Script.synthetic(Uri uri)
+      : readableUri = uri,
+        resourceUri = uri,
+        file = new StringSourceFile.fromUri(uri,
+            "// Synthetic source file generated for '$uri'."),
+        isSynthesized = true;
 
   String get text => (file == null) ? null : file.slowText();
   String get name => (file == null) ? null : file.filename;
 
   /// Creates a new [Script] with the same URIs, but new content ([file]).
   Script copyWithFile(SourceFile file) {
     return new Script(readableUri, resourceUri, file);
   }
 }