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

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

Index: pkg/compiler/lib/src/ssa/optimize.dart
diff --git a/pkg/compiler/lib/src/ssa/optimize.dart b/pkg/compiler/lib/src/ssa/optimize.dart
index a8d82ae15972f17dd059ea22ebce60a69f55e2a2..8d274d710f987cf95979abfbcef41b3ed4524a91 100644
--- a/pkg/compiler/lib/src/ssa/optimize.dart
+++ b/pkg/compiler/lib/src/ssa/optimize.dart
@@ -249,8 +249,7 @@ class SsaInstructionSimplifier extends HBaseVisitor
     // 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;