Avoid using FunctionSignature in builder_kernel

pkg/compiler/lib/src/kernel/element_map.dart

 // Copyright (c) 2017, 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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/names.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
@@ skipping 130 matching lines @@
 
   /// Compute the kind of foreign helper function called by [node], if any.
   ForeignKind getForeignKind(ir.StaticInvocation node);
 
   /// Computes the [InterfaceType] referenced by a call to the
   /// [JS_INTERCEPTOR_CONSTANT] function, if any.
   InterfaceType getInterfaceTypeForJsInterceptorCall(ir.StaticInvocation node);
 
   /// Computes the [ConstantValue] for the constant [expression].
   ConstantValue getConstantValue(ir.Expression expression,
-      {bool requireConstant: true});
+      {bool requireConstant: true, bool implicitNull: false});
 
   /// Returns the `noSuchMethod` [FunctionEntity] call from a
   /// `super.noSuchMethod` invocation within [cls].
   FunctionEntity getSuperNoSuchMethod(ClassEntity cls);
 
   /// Returns a [Spannable] for a message pointing to the IR [node] in the
   /// context of [member].
   Spannable getSpannable(MemberEntity member, ir.Node node);
 }
 
@@ skipping 74 matching lines @@
     return new Selector.getter(name);
   }
 
   Selector getSetterSelector(ir.Name irName) {
     Name name = new Name(
         irName.name, irName.isPrivate ? getLibrary(irName.library) : null);
     return new Selector.setter(name);
   }
 
   ConstantValue getConstantValue(ir.Expression node,
-      {bool requireConstant: true}) {
-    ConstantExpression constant =
-        new Constantifier(this, requireConstant: requireConstant).visit(node);
+      {bool requireConstant: true, bool implicitNull: false}) {
+    ConstantExpression constant;
+    if (node == null) {
+      if (!implicitNull) {
+        throw new SpannableAssertionFailure(
+            CURRENT_ELEMENT_SPANNABLE, 'No expression for constant.');
+      }
+      constant = new NullConstantExpression();
+    } else {
+      constant =
+          new Constantifier(this, requireConstant: requireConstant).visit(node);
+    }
     if (constant == null) {
       if (requireConstant) {
         throw new UnsupportedError(
             'No constant for ${DebugPrinter.prettyPrint(node)}');
       }
       return null;
     }
     return computeConstantValue(constant, requireConstant: requireConstant);
   }
 
@@ skipping 752 matching lines @@
   /// Call to notify that [member] is no longer being inlined.
   void leaveInlinedMember(MemberEntity member);
 
   /// Returns the [Local] for [node].
   Local getLocal(ir.VariableDeclaration node);
 
   /// Returns the [JumpTarget] for the branch in [node].
   // TODO(johnniwinther): Split this by kind of [node]?
   JumpTarget getJumpTarget(ir.TreeNode node, {bool isContinueTarget: false});
 }