Replace TypeMask.isEmpty by isEmptyOrNull, use isEmpty when it's really empty.

pkg/compiler/lib/src/types/container_type_mask.dart

Index: pkg/compiler/lib/src/types/container_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/container_type_mask.dart b/pkg/compiler/lib/src/types/container_type_mask.dart
index 5f224627923bb986c1faf264317d03621d4b538a..c38a03ab5bc8bf2920f7c27c6d0e6810279f6215 100644
--- a/pkg/compiler/lib/src/types/container_type_mask.dart
+++ b/pkg/compiler/lib/src/types/container_type_mask.dart
@@ -61,7 +61,7 @@ class ContainerTypeMask extends ForwardingTypeMask {
 
   TypeMask intersection(TypeMask other, ClassWorld classWorld) {
     TypeMask forwardIntersection = forwardTo.intersection(other, classWorld);
-    if (forwardIntersection.isEmpty) return forwardIntersection;
+    if (forwardIntersection.isEmptyOrNull) return forwardIntersection;
     return forwardIntersection.isNullable
         ? nullable()
         : nonNullable();
@@ -72,7 +72,7 @@ class ContainerTypeMask extends ForwardingTypeMask {
       return this;
     } else if (equalsDisregardNull(other)) {
       return other.isNullable ? other : this;
-    } else if (other.isEmpty) {
+    } else if (other.isEmptyOrNull) {
       return other.isNullable ? this.nullable() : this;
     } else if (other.isContainer
                && elementType != null