Make isEmpty a getter.

lib/compiler/implementation/world.dart

Index: lib/compiler/implementation/world.dart
diff --git a/lib/compiler/implementation/world.dart b/lib/compiler/implementation/world.dart
index ee91eef1fee3e96d527713295637c6764e548d49..e79b01606d230fe7df7262358a0d57168338d050 100644
--- a/lib/compiler/implementation/world.dart
+++ b/lib/compiler/implementation/world.dart
@@ -40,7 +40,7 @@ class World {
     // (3) subclasses of (2) and (3).
 
     void potentiallyAddForRti(ClassElement cls, Function callback) {
-      if (cls.typeVariables.isEmpty()) return;
+      if (cls.typeVariables.isEmpty) return;
       if (classesNeedingRti.contains(cls)) return;
       classesNeedingRti.add(cls);
       if (callback != null) {
@@ -56,7 +56,7 @@ class World {
     compiler.resolverWorld.isChecks.forEach((DartType type) {
       if (type is InterfaceType) {
         InterfaceType itf = type;
-        if (!itf.arguments.isEmpty()) {
+        if (!itf.arguments.isEmpty) {
           potentiallyAddForRti(itf.element, null);
         }
       }
@@ -64,7 +64,7 @@ class World {
 
     List<ClassElement> worklist =
         new List<ClassElement>.from(classesNeedingRti);
-    while (!worklist.isEmpty()) {
+    while (!worklist.isEmpty) {
       Element e = worklist.removeLast();
       Set<Element> dependencies = rtiDependencies[e];
       if (dependencies == null) continue;
@@ -189,5 +189,5 @@ class MemberSet {
     elements.add(element);
   }
 
-  bool isEmpty() => elements.isEmpty();
+  bool get isEmpty => elements.isEmpty;
 }