dart2dart: Fix bad type annotation in backend_ast_emitter.

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

Index: pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart b/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
index 01a9ae0869e91d4bcf6a753ea9d3e2ff6186e991..277966315b3c6ee24c63c05a598664cff5034e28 100644
--- a/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
+++ b/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)) {