Make isEmpty a getter.

tests/language/compile_time_constant_b_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 const m1 = const { '__proto__': 400 + 99 };
 const m2 = const { 'a': 499, 'b': 42 };
 const m3 = const { '__proto__': 499 };
 
 bool isUnsupportedError(o) => o is UnsupportedError;
 
 main() {
   Expect.equals(499, m1['__proto__']);
   Expect.equals(null, m1['b']);
   Expect.listEquals(['__proto__'], m1.getKeys());
   Expect.listEquals([499], m1.getValues());
   Expect.isTrue(m1.containsKey('__proto__'));
   Expect.isFalse(m1.containsKey('toString'));
   Expect.isTrue(m1.containsValue(499));
   Expect.isFalse(m1.containsValue(null));
   var seenKeys = [];
   var seenValues = [];
   m1.forEach((key, value) {
     seenKeys.add(key);
     seenValues.add(value);
   });
   Expect.listEquals(['__proto__'], seenKeys);
   Expect.listEquals([499], seenValues);
-  Expect.isFalse(m1.isEmpty());
+  Expect.isFalse(m1.isEmpty);
   Expect.equals(1, m1.length);
   Expect.throws(() => m1.remove('__proto__'), isUnsupportedError);
   Expect.throws(() => m1.remove('b'), isUnsupportedError);
   Expect.throws(() => m1.clear(), isUnsupportedError);
   Expect.throws(() => m1['b'] = 42, isUnsupportedError);
   Expect.throws(() => m1['__proto__'] = 499, isUnsupportedError);
   Expect.throws(() => m1.putIfAbsent('__proto__', () => 499),
                 isUnsupportedError);
   Expect.throws(() => m1.putIfAbsent('z', () => 499), isUnsupportedError);
 
@@ skipping 11 matching lines @@
   Expect.isTrue(m2.containsValue(42));
   Expect.isFalse(m2.containsValue(null));
   seenKeys = [];
   seenValues = [];
   m2.forEach((key, value) {
     seenKeys.add(key);
     seenValues.add(value);
   });
   Expect.listEquals(['a', 'b'], seenKeys);
   Expect.listEquals([499, 42], seenValues);
-  Expect.isFalse(m2.isEmpty());
+  Expect.isFalse(m2.isEmpty);
   Expect.equals(2, m2.length);
   Expect.throws(() => m2.remove('a'), isUnsupportedError);
   Expect.throws(() => m2.remove('b'), isUnsupportedError);
   Expect.throws(() => m2.remove('__proto__'), isUnsupportedError);
   Expect.throws(() => m2.clear(), isUnsupportedError);
   Expect.throws(() => m2['a'] = 499, isUnsupportedError);
   Expect.throws(() => m2['b'] = 42, isUnsupportedError);
   Expect.throws(() => m2['__proto__'] = 499, isUnsupportedError);
   Expect.throws(() => m2.putIfAbsent('a', () => 499), isUnsupportedError);
   Expect.throws(() => m2.putIfAbsent('__proto__', () => 499),
                 isUnsupportedError);
   Expect.throws(() => m2['a'] = 499, isUnsupportedError);
 
   Expect.isTrue(m1 === m3);
 }