Revert "Revert "Build new IR for functions returning a constant.""

sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart b/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
index 1eed9f52c74e1f34d71d51d709ae1a574b3f8be9..58fa571b419ca431365667d006cd4c623edde538 100644
--- a/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/inferrer/type_graph_inferrer.dart
@@ -15,6 +15,7 @@ import 'inferrer_visitor.dart' show TypeSystem, ArgumentsTypes;
 import '../native_handler.dart' as native;
 import '../util/util.dart' show Spannable, Setlet;
 import 'simple_types_inferrer.dart';
+import 'ir_type_inferrer.dart';
 import '../dart2jslib.dart' show invariant;
 
 part 'type_graph_nodes.dart';
@@ -472,8 +473,12 @@ class TypeGraphInferrerEngine
     if (analyzedElements.contains(element)) return;
     analyzedElements.add(element);
 
-    SimpleTypeInferrerVisitor visitor =
-        new SimpleTypeInferrerVisitor(element, compiler, this);
+      var visitor;
+      if (compiler.irBuilder.hasIr(element)) {
+        visitor = new IrTypeInferrerVisitor(compiler, element, this);
+      } else {
+        visitor = new SimpleTypeInferrerVisitor(element, compiler, this);
+      }
     TypeInformation type;
     compiler.withCurrentElement(element, () {
       type = visitor.run();
@@ -684,6 +689,7 @@ class TypeGraphInferrerEngine
   void recordReturnType(Element element, TypeInformation type) {
     TypeInformation info = types.getInferredTypeOf(element);
     if (element.name == '==') {
+      // Even if x.== doesn't return a bool, 'x == null' evaluates to 'false'.
       info.addAssignment(types.boolType);
     }
     // TODO(ngeoffray): Clean up. We do these checks because