Fix unparsing of prefix increments.

lib/compiler/implementation/tree/unparser.dart

Index: lib/compiler/implementation/tree/unparser.dart
diff --git a/lib/compiler/implementation/tree/unparser.dart b/lib/compiler/implementation/tree/unparser.dart
index 0e1179b9a28f3b6b0b5a8bee3a3c472a9257e0bb..160890e179a476d2331e605ad765b75ac94161aa 100644
--- a/lib/compiler/implementation/tree/unparser.dart
+++ b/lib/compiler/implementation/tree/unparser.dart
@@ -224,11 +224,16 @@ class Unparser implements Visitor {
    * Special case for assignments like "list[0] = 1".
    */
   unparseIndexedSet(SendSet node) {
+    if (node.isPrefix) {
+      add(node.assignmentOperator.token.value);
+    }
     visit(node.receiver);
     sb.add('[');
     sb.add(node.arguments.head);
     sb.add(']');
-    add(node.assignmentOperator.token.value);
+    if (!node.isPrefix) {
+      add(node.assignmentOperator.token.value);
+    }
     unparseNodeListFrom(node.argumentsNode, node.argumentsNode.nodes.tail);
   }
 
@@ -236,8 +241,13 @@ class Unparser implements Visitor {
     if (node.isIndex) {
       unparseIndexedSet(node);
     } else {
+      if (node.isPrefix) {
+        add(node.assignmentOperator.token.value);
+      }
       unparseSendPart(node);
-      add(node.assignmentOperator.token.value);
+      if (!node.isPrefix) {
+        add(node.assignmentOperator.token.value);
+      }
       visit(node.argumentsNode);
     }
   }