Fix yield for dart2dart minified.

pkg/compiler/lib/src/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.
 
 part of tree;
 
 String unparse(Node node, {minify: true}) {
   Unparser unparser = new Unparser(minify: minify);
   unparser.unparse(node);
   return unparser.result;
@@ skipping 390 matching lines @@
     if (node.beginToken.stringValue == '=>') space();
     visit(node.expression);
     if (node.endToken != null) write(node.endToken.value);
   }
 
   visitYield(Yield node) {
     write(node.yieldToken.value);
     if (node.starToken != null) {
       write(node.starToken.value);
     }
-    space();
+    write(' ');
     visit(node.expression);
     write(node.endToken.value);
   }
 
 
   unparseSendReceiver(Send node, {bool spacesNeeded: false}) {
     if (node.receiver == null) return;
     visit(node.receiver);
     CascadeReceiver asCascadeReceiver = node.receiver.asCascadeReceiver();
     if (asCascadeReceiver != null) {
@@ skipping 418 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }