Introduce OpenWorld.

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 8b5a5f5df105ec049b0dd4a7e23be16f46367c6f..aa83fbed981fc174f531d840facb1a633a064a1e 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -77,7 +77,7 @@ import 'universe/universe.dart' show ResolutionUniverse, CodegenUniverse;
 import 'universe/use.dart' show StaticUse;
 import 'universe/world_impact.dart' show ImpactStrategy, WorldImpact;
 import 'util/util.dart' show Link, Setlet;
-import 'world.dart' show ClosedWorld, ClosedWorldRefiner, World;
+import 'world.dart' show ClosedWorld, ClosedWorldRefiner, OpenWorld, WorldImpl;
 
 typedef Backend MakeBackendFuncion(Compiler compiler);
 
@@ -88,7 +88,7 @@ abstract class Compiler implements LibraryLoaderListener {
   Measurer get measurer;
 
   final IdGenerator idGenerator = new IdGenerator();
-  World _world;
+  WorldImpl _world;
   Types types;
   _CompilerCoreTypes _coreTypes;
   CompilerDiagnosticReporter _reporter;
@@ -225,7 +225,7 @@ abstract class Compiler implements LibraryLoaderListener {
         this.userOutputProvider = outputProvider == null
             ? const NullCompilerOutput()
             : outputProvider {
-    _world = new World(this);
+    _world = new WorldImpl(this);
     if (makeReporter != null) {
       _reporter = makeReporter(this, options);
     } else {
@@ -304,7 +304,7 @@ abstract class Compiler implements LibraryLoaderListener {
 
   /// The world currently being computed by resolution. This forms a basis for
   /// the [inferenceWorld] and later the [closedWorld].
-  World get openWorld => _world;
+  OpenWorld get openWorld => _world;
 
   /// The closed world after resolution but currently refined by inference.
   ClosedWorldRefiner get inferenceWorld => _world;