Revert "Allow import/export of libraries with no name."

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 1342ce8cbe080f457dcb87324ba557ead384517b..be9e3b76a8ca713d9de25f05feb19ecfb5fba9c1 100644
--- a/sdk/lib/_internal/compiler/implementation/library_loader.dart
+++ b/sdk/lib/_internal/compiler/implementation/library_loader.dart
@@ -421,6 +421,15 @@ class LibraryLoaderTask extends LibraryLoader {
           if (loadedLibrary == null) return;
           compiler.withCurrentElement(library, () {
             handler.registerDependency(library, tag, loadedLibrary);
+
+            if (!loadedLibrary.hasLibraryName()) {
+              compiler.reportFatalError(
+                  tag == null ? null : tag.uri,
+                  MessageKind.GENERIC,
+                  {'text':
+                   'Error: No library name found in '
+                      '${loadedLibrary.canonicalUri}.'});
+            }
           });
         });
   }