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

pkg/compiler/lib/src/script.dart

Index: pkg/compiler/lib/src/script.dart
diff --git a/pkg/compiler/lib/src/script.dart b/pkg/compiler/lib/src/script.dart
index 3dce8e189ba4240f9f0e9042318aca929712a17d..f8278ab248923c5323a2fde94ca95f221c56c96b 100644
--- a/pkg/compiler/lib/src/script.dart
+++ b/pkg/compiler/lib/src/script.dart
@@ -27,9 +27,15 @@ class Script {
   /// 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;