Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

pkg/compiler/lib/src/universe/world_builder.dart

Index: pkg/compiler/lib/src/universe/world_builder.dart
diff --git a/pkg/compiler/lib/src/universe/world_builder.dart b/pkg/compiler/lib/src/universe/world_builder.dart
index 130b7eb4e9b7e138a185f8a53110431deb82d53d..cd676bdc659516be8858ced85e430770d49e5009 100644
--- a/pkg/compiler/lib/src/universe/world_builder.dart
+++ b/pkg/compiler/lib/src/universe/world_builder.dart
@@ -172,6 +172,11 @@ abstract class ResolutionWorldBuilder implements WorldBuilder {
   /// The [OpenWorld] being created by this world builder.
   // TODO(johnniwinther): Merge this with [ResolutionWorldBuilder].
   OpenWorld get openWorld;
+
+  /// The closed world computed by this world builder.
+  ///
+  /// This is only available after the world builder has been closed.
+  ClosedWorld get closedWorldForTesting;
 }
 
 /// The type and kind of an instantiation registered through
@@ -425,7 +430,7 @@ class ResolutionWorldBuilderImpl implements ResolutionWorldBuilder {
   /// and classes.
   bool useInstantiationMap = false;
 
-  OpenWorld _openWorld;
+  WorldImpl _openWorld;
 
   final Backend _backend;
   final Resolution _resolution;
@@ -443,6 +448,14 @@ class ResolutionWorldBuilderImpl implements ResolutionWorldBuilder {
 
   OpenWorld get openWorld => _openWorld;
 
+  ClosedWorld get closedWorldForTesting {
+    if (!_openWorld.isClosed) {
+      throw new SpannableAssertionFailure(
+          NO_LOCATION_SPANNABLE, "The world builder has not yet been closed.");
+    }
+    return _openWorld;
+  }
+
   /// All directly instantiated classes, that is, classes with a generative
   /// constructor that has been called directly and not only through a
   /// super-call.