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

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

Index: pkg/compiler/lib/src/types/map_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/map_type_mask.dart b/pkg/compiler/lib/src/types/map_type_mask.dart
index 39ed50bf613d5cf13c89023e9ff759dbc3124ace..52fe4a2a7af77ae05fe8abd7b3207754d71b9d95 100644
--- a/pkg/compiler/lib/src/types/map_type_mask.dart
+++ b/pkg/compiler/lib/src/types/map_type_mask.dart
@@ -64,7 +64,7 @@ class MapTypeMask 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();
@@ -75,7 +75,7 @@ class MapTypeMask 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.isMap &&
                keyType != null &&