Remove the need for functionSignature.

sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_nodes.dart

 // Copyright (c) 2014, 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.
 
 library tree_ir_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values;
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
 import '../dart_types.dart' show DartType, GenericType;
@@ skipping 265 matching lines @@
 
   Not(this.operand);
 
   accept(ExpressionVisitor visitor) => visitor.visitNot(this);
 }
 
 class FunctionExpression extends Expression {
   final FunctionDefinition definition;
 
   FunctionExpression(this.definition) {
-    assert(definition.element.functionSignature.type.returnType.treatAsDynamic);
+    assert(definition.element.type.returnType.treatAsDynamic);
   }
 
   accept(ExpressionVisitor visitor) => visitor.visitFunctionExpression(this);
 }
 
 /// Declares a local function.
 /// Used for functions that may not occur in expression context due to
 /// being recursive or having a return type.
 /// The [variable] must not occur as the left-hand side of an [Assign] or
 /// any other [FunctionDeclaration].
@@ skipping 350 matching lines @@
   visitFunctionDeclaration(FunctionDeclaration node) {
     visitFunctionDefinition(node.definition);
     visitStatement(node.next);
   }
 
   visitExpressionStatement(ExpressionStatement node) {
     visitExpression(node.expression);
     visitStatement(node.next);
   }
 }