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

pkg/compiler/lib/src/ssa/nodes.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 abstract class HVisitor<R> {
   R visitAdd(HAdd node);
   R visitAwait(HAwait node);
   R visitBitAnd(HBitAnd node);
@@ skipping 863 matching lines @@
 
   /// Does this node potentially affect control flow.
   bool isControlFlow() => false;
 
   bool isExact() => instructionType.isExact || isNull();
 
   bool isValue() => instructionType.isValue;
 
   bool canBeNull() => instructionType.isNullable;
 
-  bool isNull() => instructionType.isEmpty && instructionType.isNullable;
-  bool isConflicting() {
-    return instructionType.isEmpty && !instructionType.isNullable;
-  }
+  bool isNull() => instructionType.isNull;
+
+  bool isConflicting() => instructionType.isEmpty;
 
   /// Returns `true` if [typeMask] contains [cls].
   static bool containsType(
       TypeMask typeMask,
       ClassElement cls,
       ClassWorld classWorld) {
     return classWorld.isInstantiated(cls) && typeMask.contains(cls, classWorld);
   }
 
   /// Returns `true` if [typeMask] contains only [cls].
@@ skipping 2444 matching lines @@
 class HDynamicType extends HRuntimeType {
   HDynamicType(DynamicType dartType, TypeMask instructionType)
       : super(const <HInstruction>[], dartType, instructionType);
 
   accept(HVisitor visitor) => visitor.visitDynamicType(this);
 
   int typeCode() => HInstruction.DYNAMIC_TYPE_TYPECODE;
 
   bool typeEquals(HInstruction other) => other is HDynamicType;
 }