Handle exception from scanning.

sdk/lib/_internal/compiler/implementation/library_loader.dart

Index: sdk/lib/_internal/compiler/implementation/library_loader.dart
diff --git a/sdk/lib/_internal/compiler/implementation/library_loader.dart b/sdk/lib/_internal/compiler/implementation/library_loader.dart
index 3c515a18c270cb52ced6910fc845255eaac0e1f9..e405dcfd56fa36ca2dd850326aad8590228ec0e3 100644
--- a/sdk/lib/_internal/compiler/implementation/library_loader.dart
+++ b/sdk/lib/_internal/compiler/implementation/library_loader.dart
@@ -281,7 +281,7 @@ class LibraryLoaderTask extends LibraryLoader {
 
     // TODO(rnystrom): Remove .toList() here if #11523 is fixed.
     return Future.forEach(library.tags.reverse().toList(), (LibraryTag tag) {
-      compiler.withCurrentElement(library, () {
+      return compiler.withCurrentElement(library, () {
         if (tag.isImport) {
           Import import = tag;
           tagState = checkTag(TagState.IMPORT_OR_EXPORT, import);