Fix some warnings.

lib/compiler/implementation/constants.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.
 
 abstract class ConstantVisitor<R> {
   R visitSentinel(SentinelConstant constant);
   R visitFunction(FunctionConstant constant);
   R visitNull(NullConstant constant);
   R visitInt(IntConstant constant);
   R visitDouble(DoubleConstant constant);
@@ skipping 41 matching lines @@
   static final SENTINEL = const SentinelConstant();
 
   List<Constant> getDependencies() => const <Constant>[];
 
   // Just use a random value.
   int hashCode() => 24297418;
 
   bool isSentinel() => true;
 
   accept(ConstantVisitor visitor) => visitor.visitSentinel(this);
+
+  DartType computeType(Compiler compiler) => compiler.types.dynamicType;
 }
 
 class FunctionConstant extends Constant {
   Element element;
 
   FunctionConstant(this.element);
 
   bool isFunction() => true;
 
   bool operator ==(var other) {
@@ skipping 362 matching lines @@
       if (fields[i] != other.fields[i]) return false;
     }
     return true;
   }
 
   int hashCode() => _hashCode;
   List<Constant> getDependencies() => fields;
 
   accept(ConstantVisitor visitor) => visitor.visitConstructed(this);
 }