Support ExpressionStatement in the dart2dart backend.

sdk/lib/_internal/compiler/implementation/ir/ir_nodes.dart

 // Copyright (c) 2013, 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.
 
 // IrNodes are kept in a separate library to have precise control over their
 // dependencies on other parts of the system.
 library dart2js.ir_nodes;
 
 import '../dart2jslib.dart' as dart2js show Constant;
 import '../elements/elements.dart' show FunctionElement, LibraryElement;
@@ skipping 13 matching lines @@
 }
 
 /// The base class of things that variables can refer to: primitives,
 /// continuations, function and continuation parameters, etc.
 abstract class Definition extends Node {
   // The head of a linked-list of occurrences, in no particular order.
   Reference firstRef = null;
 
   bool get hasAtMostOneUse => firstRef == null || firstRef.nextRef == null;
   bool get hasExactlyOneUse => firstRef != null && firstRef.nextRef == null;
+  bool get hasAtLeastOneUse => firstRef != null;
 }
 
 abstract class Primitive extends Definition {
 }
 
 /// Operands to invocations and primitives are always variables.  They point to
 /// their definition and are linked into a list of occurrences.
 class Reference {
   Definition definition;
   Reference nextRef = null;
@@ skipping 203 matching lines @@
   String visitParameter(Parameter triv) {
     // Parameters are visited directly in visitLetCont.
     return '(Unexpected Parameter)';
   }
 
   String visitContinuation(Continuation triv) {
     // Continuations are visited directly in visitLetCont.
     return '(Unexpected Continuation)';
   }
 }