Make isEmpty a getter.

lib/compiler/implementation/ssa/validate.dart

Index: lib/compiler/implementation/ssa/validate.dart
diff --git a/lib/compiler/implementation/ssa/validate.dart b/lib/compiler/implementation/ssa/validate.dart
index 94bc1369bf2cf52d61bc8068e1c93523dbc13898..aa44966ea54d02ef60393c133449d8f4cde13d9b 100644
--- a/lib/compiler/implementation/ssa/validate.dart
+++ b/lib/compiler/implementation/ssa/validate.dart
@@ -46,14 +46,14 @@ class HValidator extends HInstructionVisitor {
         (block.successors.length != 1 || !block.successors[0].isExitBlock())) {
       markInvalid("Return node with > 1 succesor or not going to exit-block");
     }
-    if (block.last is HExit && !block.successors.isEmpty()) {
+    if (block.last is HExit && !block.successors.isEmpty) {
       markInvalid("Exit block with successor");
     }
-    if (block.last is HThrow && !block.successors.isEmpty()) {
+    if (block.last is HThrow && !block.successors.isEmpty) {
       markInvalid("Throw block with successor");
     }
 
-    if (block.successors.isEmpty() &&
+    if (block.successors.isEmpty &&
         block.last is !HThrow &&
         !block.isExitBlock()) {
       markInvalid("Non-exit or throw block without successor");