Incremental compilation of libraries with multiple parts.

dart/pkg/compiler/lib/src/script.dart

Index: dart/pkg/compiler/lib/src/script.dart
diff --git a/dart/pkg/compiler/lib/src/script.dart b/dart/pkg/compiler/lib/src/script.dart
index 9ceafb468094b622877a28b7e9c45a18487293f6..54bd63153254e7867f878e27d72e10211eaa71b6 100644
--- a/dart/pkg/compiler/lib/src/script.dart
+++ b/dart/pkg/compiler/lib/src/script.dart
@@ -26,4 +26,9 @@ class Script {
 
   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);
+  }
 }