Parse/unparse library combinators.

dart/lib/compiler/implementation/scanner/scanner_task.dart

Index: dart/lib/compiler/implementation/scanner/scanner_task.dart
diff --git a/dart/lib/compiler/implementation/scanner/scanner_task.dart b/dart/lib/compiler/implementation/scanner/scanner_task.dart
index 28701c990144dee98565a61e2e50bad14c2680f9..899bf39c5cd3a8a82670f2a12cedb5cc6b76365f 100644
--- a/dart/lib/compiler/implementation/scanner/scanner_task.dart
+++ b/dart/lib/compiler/implementation/scanner/scanner_task.dart
@@ -40,6 +40,9 @@ class ScannerTask extends CompilerTask {
     for (LibraryTag tag in library.tags.reverse()) {
       if (tag.isImport) {
         tagState = checkTag(TagState.IMPORT, tag);
+        if (tag.combinators != null) {
+          compiler.unimplemented('combinators', node: tag.combinators);
+        }
         // It is not safe to import other libraries at this point as
         // another library could then observe the current library
         // before it fully declares all the members that are sourced