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

pkg/compiler/lib/src/ssa/optimize.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 OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 231 matching lines @@
   HInstruction visitBoolify(HBoolify node) {
     List<HInstruction> inputs = node.inputs;
     assert(inputs.length == 1);
     HInstruction input = inputs[0];
     if (input.isBoolean(compiler)) return input;
 
     // If the code is unreachable, remove the HBoolify.  This can happen when
     // there is a throw expression in a short-circuit conditional.  Removing the
     // unreachable HBoolify makes it easier to reconstruct the short-circuit
     // operation.
-    if (input.instructionType.isEmpty && !input.instructionType.isNullable)
-      return input;
+    if (input.instructionType.isEmpty) return input;
 
     // All values that cannot be 'true' are boolified to false.
     TypeMask mask = input.instructionType;
     if (!mask.contains(helpers.jsBoolClass, compiler.world)) {
       return graph.addConstantBool(false, compiler);
     }
     return node;
   }
 
   HInstruction visitNot(HNot node) {
@@ skipping 2134 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }