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

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

Index: pkg/compiler/lib/src/types/dictionary_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/dictionary_type_mask.dart b/pkg/compiler/lib/src/types/dictionary_type_mask.dart
index 27419779d58396fd34e9d5f4e5520016c7c2dde6..cba828f7ad852bc692c1b2c02075cf889bd4a8da 100644
--- a/pkg/compiler/lib/src/types/dictionary_type_mask.dart
+++ b/pkg/compiler/lib/src/types/dictionary_type_mask.dart
@@ -59,7 +59,7 @@ class DictionaryTypeMask extends MapTypeMask {
 
   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();
@@ -70,7 +70,7 @@ class DictionaryTypeMask extends MapTypeMask {
       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.isDictionary) {
       TypeMask newForwardTo = forwardTo.union(other.forwardTo, classWorld);
@@ -115,4 +115,4 @@ class DictionaryTypeMask extends MapTypeMask {
     return
         'Dictionary mask: [$keyType/$valueType with $typeMap] type: $forwardTo';
   }
-}
+}