Support abstract getters without parameters.

dart/lib/compiler/implementation/compile_time_constants.dart

Index: dart/lib/compiler/implementation/compile_time_constants.dart
diff --git a/dart/lib/compiler/implementation/compile_time_constants.dart b/dart/lib/compiler/implementation/compile_time_constants.dart
index c0e428ab1e7df953dc6841fe307e89742edf20fc..32064cbdaceef160216a3f87288de4d6c94c75c1 100644
--- a/dart/lib/compiler/implementation/compile_time_constants.dart
+++ b/dart/lib/compiler/implementation/compile_time_constants.dart
@@ -34,7 +34,7 @@ class Constant implements Hashable {
 }
 
 class PrimitiveConstant extends Constant {
-  abstract get value();
+  abstract get value;
   const PrimitiveConstant();
   bool isPrimitive() => true;
 
@@ -74,7 +74,7 @@ class NullConstant extends PrimitiveConstant {
 }
 
 class NumConstant extends PrimitiveConstant {
-  abstract num get value();
+  abstract num get value;
   const NumConstant();
   bool isNum() => true;
 }