Complain about duplicated defer.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index 24b8c29c606c86215731918157a930752e40147a..fbf6d7000f9f3b41f800f69619d44113b44d6bf5 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -280,6 +280,16 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
     builder.next = existing;
     if (builder is PrefixBuilder && existing is PrefixBuilder) {
       assert(existing.next == null);
+      if (builder.deferred) {
+        addCompileTimeError(builder.charOffset,
+            "Can't use the name '$name' for a deferred library, as the name is used elsewhere.");

[Johnni Winther, 2017/06/12 08:09:13]

Long line + below

[ahe, 2017/06/12 08:33:28]

Acknowledged.

+        addCompileTimeError(existing.charOffset, "'$name' is used here.");
+      }
+      if (existing.deferred) {

[Johnni Winther, 2017/06/12 08:09:13]

Make this an `else if` to avoid to errors when both are deferred. Or maybe
special case that situation?

[ahe, 2017/06/12 08:33:28]

Acknowledged.

+        addCompileTimeError(existing.charOffset,
+            "Can't use the name '$name' for a deferred library, as the name is used elsewhere.");
+        addCompileTimeError(builder.charOffset, "'$name' is used here.");
+      }
       return existing
         ..exports.merge(builder.exports,
             (String name, Builder existing, Builder member) {