dart2js: Type inferrence: Account for implicit null test in '??' and '??='.

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

Index: pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
index 61f8595ff3abda9e54aa8e54679c6720a49522ad..2ae18625ea0b1e1d7b76940aa7e08d707483be0d 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -83,6 +83,10 @@ class TypeMaskSystem implements TypeSystem<TypeMask> {
     return type.intersection(otherType, classWorld);
   }
 
+  TypeMask narrowNotNull(TypeMask type) {
+    return type.nonNullable();
+  }
+
   TypeMask computeLUB(TypeMask firstType, TypeMask secondType) {
     if (firstType == null) {
       return secondType;