dart2dart: Fix bad type annotation in backend_ast_emitter.

Issue: https://code.google.com/p/dart/issues/detail?id=22326

R=kmillikin@google.com

Review URL: https://codereview.chromium.org//911573002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@43592 260f80e4-7a28-3924-810f-c04153c831b5

pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart

@@ -434,7 +434,7 @@ class ASTEmitter
   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)) {