Make the universe parts into small libraries.

tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart

Index: tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
diff --git a/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart b/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
index 736faf00895e42e65b6b6039319654152116e9e7..e0252e71159c4d3735b67090c163717914e393a2 100644
--- a/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
+++ b/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
@@ -17,8 +17,10 @@ import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/modelx.dart'
     show ErroneousElementX, TypeVariableElementX;
 import 'package:compiler/src/tree/tree.dart' show LiteralDartString;
-import 'package:compiler/src/universe/universe.dart'
-    show Selector, SelectorKind, CallStructure;
+import 'package:compiler/src/universe/call_structure.dart'
+    show CallStructure;
+import 'package:compiler/src/universe/selector.dart'
+    show Selector, SelectorKind;
 import 'package:compiler/src/cps_ir/cps_ir_nodes.dart';
 
 /// Used whenever a node constructed by [SExpressionUnstringifier] needs a