Make isEmpty a getter.

tests/co19/co19-compiler.status

 # 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.
 
 [ $compiler == dartc ]
 # We are now actually skipping these because of inter-test dependencies.
 Language/13_Libraries_and_Scripts/3_Includes_A01_t23: Skip # TODO(analyzer-team): Please triage this crash.
 
 Language/03_Overview/1_Scoping_A02_t06: Fail # TODO(analyzer-team): Please triage this failure.
 Language/03_Overview/2_Privacy_A01_t04: Fail # TODO(analyzer-team): Please triage this failure.
@@ skipping 192 matching lines @@
 Language/10_Expressions/21_Bitwise_Expressions_A01_t14: Fail # language change 3368
 Language/10_Expressions/21_Bitwise_Expressions_A01_t15: Fail # language change 3368
 Language/10_Expressions/22_Equality_A01_t19: Fail # language change 3368
 Language/10_Expressions/22_Equality_A01_t20: Fail # language change 3368
 Language/10_Expressions/22_Equality_A02_t03: Fail, OK # co19 issue 169
 Language/10_Expressions/23_Relational_Expressions_A01_t20: Fail # language change 3368
 Language/10_Expressions/23_Relational_Expressions_A01_t21: Fail # language change 3368
 Language/10_Expressions/24_Shift_A01_t11: Fail # language change 3368
 Language/10_Expressions/24_Shift_A01_t12: Fail # language change 3368
 
-
 # The following tests use hashCode() (function) instead of hashCode (getter).
 # co19 issue 273
 Language/10_Expressions/11_Instance_Creation/1_New_A12_t01: Fail, OK
 LibTest/isolate/SendPort/hashCode_A01_t01: Pass, OK
 LibTest/core/Map/putIfAbsent_A01_t04: Fail, OK
 LibTest/core/Map/Map_class_A01_t03: Fail, OK
 LibTest/core/Set/remove_A01_t03: Fail, OK
 LibTest/core/Set/addAll_A01_t03: Fail, OK
 LibTest/core/Set/forEach_A01_t03: Fail, OK
 LibTest/core/Set/add_A01_t03: Fail, OK
@@ skipping 84 matching lines @@
 Language/11_Statements/05_For/2_For_in_A01_t04: Fail, OK # co19 issue 286
 Language/11_Statements/05_For/2_For_in_A01_t02: Fail, OK # co19 issue 286
 Language/11_Statements/05_For/2_For_in_A01_t05: Fail, OK # co19 issue 286
 Language/11_Statements/06_For/2_For_in_A01_t05: Fail, OK # co19 issue 286
 LibTest/core/Queue/Queue.from_A01_t02: Fail, OK # co19 issue 286
 LibTest/core/Set/Set.from_A01_t02: Fail, OK # co19 issue 286
 
 
 [ $runtime == drt && ($compiler == none || $compiler == frog) ]
 *: Skip