Fix unparsing of prefix increments.

lib/compiler/implementation/tree/unparser.dart

 // Copyright (c) 2012, 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.
 
 class Unparser implements Visitor {
   StringBuffer sb;
 
   Unparser();
 
   String unparse(Node node) {
@@ skipping 206 matching lines @@
 
   visitSend(Send node) {
     unparseSendPart(node);
     visit(node.argumentsNode);
   }
 
   /**
    * 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);
   }
 
   visitSendSet(SendSet node) {
     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);
     }
   }
 
   visitThrow(Throw node) {
     add(node.throwToken.value);
     if (node.expression !== null) {
       sb.add(' ');
       visit(node.expression);
     }
@@ skipping 192 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }