dart2dart: Fix bad type annotation in backend_ast_emitter.

pkg/compiler/lib/src/dart_backend/backend_ast_emitter.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 backend_ast_emitter;
 
 import '../tree_ir/tree_ir_nodes.dart' as tree;
 import 'backend_ast_nodes.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
@@ skipping 416 matching lines @@
   bool isSameVariable(Receiver e1, Receiver e2) {
     return e1 is Identifier &&
            e2 is Identifier &&
            e1.element is VariableElement &&
            e1.element == e2.element;
   }
 
   Expression makeAssignment(Expression target, Expression value) {
     // Try to print as compound assignment or increment
     if (value is BinaryOperator && isCompoundableOperator(value.operator)) {
-      Expression leftOperand = value.left;
+      Receiver leftOperand = value.left;
       Expression rightOperand = value.right;
       bool valid = false;
       if (isSameVariable(target, leftOperand)) {
         valid = true;
       } else if (target is FieldExpression &&
                  leftOperand is FieldExpression &&
                  isSameVariable(target.object, leftOperand.object) &&
                  target.fieldName == leftOperand.fieldName) {
         valid = true;
       } else if (target is IndexExpression &&
@@ skipping 810 matching lines @@
       : super(name, ElementKind.VARIABLE, enclosingElement, variables, null);
 
   ExecutableElement get executableContext => enclosingElement;
 
   ExecutableElement get memberContext => executableContext.memberContext;
 
   bool get isLocal => true;
 
   LibraryElement get implementationLibrary => enclosingElement.library;
 }