Handle int/double literals in inference

pkg/compiler/lib/src/inferrer/builder.dart

Index: pkg/compiler/lib/src/inferrer/builder.dart
diff --git a/pkg/compiler/lib/src/inferrer/builder.dart b/pkg/compiler/lib/src/inferrer/builder.dart
index ca683a42ec617ca29eef9d6de9614232dd81de20..900f45376e5a9c99424617e9af3a4b27bce34854 100644
--- a/pkg/compiler/lib/src/inferrer/builder.dart
+++ b/pkg/compiler/lib/src/inferrer/builder.dart
@@ -219,7 +219,7 @@ class ElementGraphBuilder extends ast.Visitor<TypeInformation>
   }
 
   TypeInformation visitLiteralDouble(ast.LiteralDouble node) {
-    ConstantSystem constantSystem = compiler.backend.constantSystem;
+    ConstantSystem constantSystem = closedWorld.constantSystem;
     // The JavaScript backend may turn this literal into an integer at
     // runtime.
     return types.getConcreteTypeFor(
@@ -227,7 +227,7 @@ class ElementGraphBuilder extends ast.Visitor<TypeInformation>
   }
 
   TypeInformation visitLiteralInt(ast.LiteralInt node) {
-    ConstantSystem constantSystem = compiler.backend.constantSystem;
+    ConstantSystem constantSystem = closedWorld.constantSystem;
     // The JavaScript backend may turn this literal into a double at
     // runtime.
     return types.getConcreteTypeFor(