Add check for duplicate field initialization

tests/language/compile_time_constant_f_test.dart

Index: tests/language/compile_time_constant_f_test.dart
===================================================================
--- tests/language/compile_time_constant_f_test.dart	(revision 10144)
+++ tests/language/compile_time_constant_f_test.dart	(working copy)
@@ -3,11 +3,10 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class A {
-  final x = 4;
+  final x;
   const A(this.x);
   const A.named([this.x]);
   const A.named2([this.x = 2]);
-  const A.named3();
 }
 
 final a1 = const A(0);
@@ -15,7 +14,6 @@
 final a3 = const A.named(1);
 final a4 = const A.named2();
 final a5 = const A.named2(3);
-final a6 = const A.named3();
 
 main() {
   Expect.equals(0, a1.x);
@@ -23,5 +21,4 @@
   Expect.equals(1, a3.x);
   Expect.equals(2, a4.x);
   Expect.equals(3, a5.x);
-  Expect.equals(4, a6.x);
 }