Make isEmpty a getter.

lib/core/expect.dart

Index: lib/core/expect.dart
diff --git a/lib/core/expect.dart b/lib/core/expect.dart
index 2b87297d315d0484a9ee354d8b7c077f2727c9d1..bdd24eb1f29ef28588d43bc4caff3f72b131a821 100644
--- a/lib/core/expect.dart
+++ b/lib/core/expect.dart
@@ -227,12 +227,12 @@ class Expect {
     final extraSet = new Set.from(actual);
     extraSet.removeAll(expected);
 
-    if (extraSet.isEmpty() && missingSet.isEmpty()) return;
+    if (extraSet.isEmpty && missingSet.isEmpty) return;
     String msg = _getMessage(reason);
 
     StringBuffer sb = new StringBuffer("Expect.setEquals($msg) fails");
     // Report any missing items.
-    if (!missingSet.isEmpty()) {
+    if (!missingSet.isEmpty) {
       sb.add('\nExpected collection does not contain: ');
     }
 
@@ -241,7 +241,7 @@ class Expect {
     }
 
     // Report any extra items.
-    if (!extraSet.isEmpty()) {
+    if (!extraSet.isEmpty) {
       sb.add('\nExpected collection should not contain: ');
     }