Split World usage into open, inference, and closed world.

pkg/compiler/lib/src/ssa/types.dart

Index: pkg/compiler/lib/src/ssa/types.dart
diff --git a/pkg/compiler/lib/src/ssa/types.dart b/pkg/compiler/lib/src/ssa/types.dart
index 3722bf5c60a4c9549002c7f0700ebc1346e28239..922486b89664cb17c48ec0660e3c83eb48c9cab0 100644
--- a/pkg/compiler/lib/src/ssa/types.dart
+++ b/pkg/compiler/lib/src/ssa/types.dart
@@ -41,7 +41,7 @@ class TypeMaskFactory {
     var typesReturned = nativeBehavior.typesReturned;
     if (typesReturned.isEmpty) return compiler.commonMasks.dynamicType;
 
-    ClassWorld world = compiler.world;
+    ClassWorld world = compiler.closedWorld;
     CommonMasks commonMasks = compiler.commonMasks;
     CoreClasses coreClasses = compiler.coreClasses;
 
@@ -60,7 +60,7 @@ class TypeMaskFactory {
 
     TypeMask result = typesReturned
         .map(fromNativeType)
-        .reduce((t1, t2) => t1.union(t2, compiler.world));
+        .reduce((t1, t2) => t1.union(t2, compiler.closedWorld));
     assert(!result.isEmpty);
     return result;
   }