Compute export namespaces for DILL libraries.

pkg/front_end/lib/src/fasta/builder/library_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index efd77e5ed09bd5163be43e75e380fbb7566bfff5..e3067aa8ccbf980457e61dbfc39343eb82734d3b 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -19,8 +19,9 @@ import '../util/relativize.dart' show relativizeUri;
 import 'builder.dart'
     show
         Builder,
-        DynamicTypeBuilder,
         ClassBuilder,
+        DynamicTypeBuilder,
+        PrefixBuilder,
         Scope,
         ScopeBuilder,
         TypeBuilder,
@@ -88,7 +89,24 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
     }
   }
 
-  bool addToExportScope(String name, Builder member);
+  /// Returns true if the export scope was modified.
+  bool addToExportScope(String name, Builder member) {
+    if (name.startsWith("_")) return false;
+    if (member is PrefixBuilder) return false;
+    Map<String, Builder> map =
+        member.isSetter ? exports.setters : exports.local;
+    Builder existing = map[name];
+    if (existing == member) return false;
+    if (existing != null) {
+      Builder result =
+          buildAmbiguousBuilder(name, existing, member, -1, isExport: true);
+      map[name] = result;
+      return result != existing;
+    } else {
+      map[name] = member;
+    }
+    return true;
+  }
 
   void addToScope(String name, Builder member, int charOffset, bool isImport);