Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 3eee56fa326f44772774f777eca78d63c2d32abb..07d9c898f675fe849fc0ff462612de5eb5d126fe 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -78,7 +78,7 @@ import 'universe/world_impact.dart'
         WorldImpactBuilder,
         WorldImpactBuilderImpl;
 import 'util/util.dart' show Link, Setlet;
-import 'world.dart' show ClosedWorld, ClosedWorldRefiner, OpenWorld, WorldImpl;
+import 'world.dart' show ClosedWorld, ClosedWorldRefiner, WorldImpl;
 
 typedef Backend MakeBackendFuncion(Compiler compiler);
 
@@ -89,7 +89,6 @@ abstract class Compiler implements LibraryLoaderListener {
   Measurer get measurer;
 
   final IdGenerator idGenerator = new IdGenerator();
-  WorldImpl get _world => resolverWorld.openWorld;
   Types types;
   _CompilerCoreTypes _coreTypes;
   CompilerDiagnosticReporter _reporter;
@@ -276,13 +275,6 @@ abstract class Compiler implements LibraryLoaderListener {
     tasks.addAll(backend.tasks);
   }
 
-  /// The closed world after resolution and inference.
-  ClosedWorld get closedWorld {
-    assert(invariant(CURRENT_ELEMENT_SPANNABLE, _world.isClosed,
-        message: "Closed world not computed yet."));
-    return _world;
-  }
-
   /// Creates the backend.
   ///
   /// Override this to mock the backend for testing.
@@ -704,6 +696,9 @@ abstract class Compiler implements LibraryLoaderListener {
         assert(mainFunction != null);
 
         ClosedWorldRefiner closedWorldRefiner = closeResolution();
+        // TODO(johnniwinther): Make [ClosedWorld] a property of
+        // [ClosedWorldRefiner].
+        ClosedWorld closedWorld = resolverWorld.closedWorldForTesting;
 
         reporter.log('Inferring types...');
         globalInference.runGlobalTypeInference(