Extract interface of InferrerEngine and move implemation to InferrerEngineImpl

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

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 253d717a8254d82067f166979c93e0d88faa6ac0..21c3a4f070976117d8d99944ee3d0e4ca3f8e670 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -65,7 +65,7 @@ class TypeGraphInferrer implements TypesInferrer {
 
   void analyzeMain(FunctionEntity main) {
     inferrer =
-        new InferrerEngine(compiler, closedWorld, closedWorldRefiner, main);
+        new InferrerEngineImpl(compiler, closedWorld, closedWorldRefiner, main);
     inferrer.runOverAllElements();
   }