dart2js allocation tweaks

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

Index: pkg/compiler/lib/src/inferrer/type_system.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_system.dart b/pkg/compiler/lib/src/inferrer/type_system.dart
index 43f4074b1ad3e1eaf50b0599bf809fa1ac5c1ccf..72dec4b5d37ef6f65ddbbd746e04b9e2d803be41 100644
--- a/pkg/compiler/lib/src/inferrer/type_system.dart
+++ b/pkg/compiler/lib/src/inferrer/type_system.dart
@@ -325,9 +325,8 @@ class TypeSystem {
 
   ElementTypeInformation getInferredTypeOf(Element element) {
     element = element.implementation;
-    return typeInformations.putIfAbsent(element, () {
-      return new ElementTypeInformation(element, this);
-    });
+    return typeInformations[element] ??=
+        new ElementTypeInformation(element, this);
   }
 
   /**