Make isEmpty a getter.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index ec6200b0c9e04e756bcdb5540b098f4c978887c6..0faf553a33f4ff29c9b2922783dc9931b3816df8 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -88,7 +88,7 @@ class _ObjectArray<E> implements List<E> {
     return Collections.some(this, f);
   }
 
-  bool isEmpty() {
+  bool get isEmpty {
     return this.length === 0;
   }
 
@@ -230,7 +230,7 @@ class _ImmutableArray<E> implements List<E> {
     return Collections.some(this, f);
   }
 
-  bool isEmpty() {
+  bool get isEmpty {
     return this.length === 0;
   }