Follow-up to CLs 2933723002 and 2932973003.

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 fbf6d7000f9f3b41f800f69619d44113b44d6bf5..f7d8952145b1a345389ff01b5fdb6bed288945e4 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
@@ -280,15 +280,21 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
     builder.next = existing;
     if (builder is PrefixBuilder && existing is PrefixBuilder) {
       assert(existing.next == null);
+      Builder deferred;
+      Builder other;
       if (builder.deferred) {
-        addCompileTimeError(builder.charOffset,
-            "Can't use the name '$name' for a deferred library, as the name is used elsewhere.");
-        addCompileTimeError(existing.charOffset, "'$name' is used here.");
+        deferred = builder;
+        other = existing;
+      } else if (existing.deferred) {
+        deferred = existing;
+        other = builder;
       }
-      if (existing.deferred) {
-        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.");
+      if (deferred != null) {
+        addCompileTimeError(
+            deferred.charOffset,
+            "Can't use the name '$name' for a deferred library, "
+            "as the name is used elsewhere.");
+        addCompileTimeError(other.charOffset, "'$name' is used here.");

[Paul Berry, 2017/06/12 16:23:12]

Not a blocking issue for this CL, but I believe this is going to be problematic
for integration with analyzer, since analyzer sorts errors by character
position, so the user is not going to understand the meaning of the "'$name' is
used here." error.

The situation is even worse for IDE integration, where the errors are frequently
displayed as popups associated with the source code location.

[ahe, 2017/06/12 16:34:08]

I'm aware of this problem, and I'm not sure what the best solution is. Either,
each message should be worded so it can be a standalone message, or we should
add more structure to the diagnostic objects.

I hadn't really thought about how bad this is for IDE integration, so I'm glad
you mentioned it.

       }
       return existing
         ..exports.merge(builder.exports,