Add debug names to scopes.

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 162005275381264723e78ae382a7f3f7f62cf6e4..57bb2409a34374f16a6b89c35c5c4db1680f4c80 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
@@ -582,10 +582,13 @@ class DeclarationBuilder<T extends TypeBuilder> {
   final Map<ProcedureBuilder, DeclarationBuilder<T>> factoryDeclarations;
 
   DeclarationBuilder(this.members, this.setters, this.constructors,
-      this.factoryDeclarations, this.name, this.parent);
+      this.factoryDeclarations, this.name, this.parent) {
+    assert(name != null);
+  }
 
   DeclarationBuilder.library()
-      : this(<String, Builder>{}, <String, Builder>{}, null, null, null, null);
+      : this(<String, Builder>{}, <String, Builder>{}, null, null, "library",
+            null);
 
   DeclarationBuilder createNested(String name, bool hasMembers) {
     return new DeclarationBuilder<T>(
@@ -661,6 +664,6 @@ class DeclarationBuilder<T extends TypeBuilder> {
   }
 
   Scope toScope(Scope parent) {
-    return new Scope(members, setters, parent, isModifiable: false);
+    return new Scope(members, setters, parent, name, isModifiable: false);
   }
 }