fixes #221, initial sync*, async, async* implementation

lib/src/js/printer.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 js_ast;
 
 
 class JavaScriptPrintingOptions {
   final bool shouldCompressOutput;
   final bool minifyLocalVariables;
@@ skipping 763 matching lines @@
       default:
         out(op);
     }
     visitNestedExpression(unary.argument, unary.precedenceLevel,
                           newInForInit: inForInit, newAtStatementBegin: false);
   }
 
   visitSpread(Spread unary) => visitPrefix(unary);
 
   visitYield(Yield yield) {
-    out(yield.star ? "yield* " : "yield ");
+    out(yield.star ? "yield*" : "yield");
+    if (yield.value == null) return;
+    out(" ");
     visitNestedExpression(yield.value, yield.precedenceLevel,
         newInForInit: inForInit, newAtStatementBegin: false);
   }
 
   visitPostfix(Postfix postfix) {
     visitNestedExpression(postfix.argument, LEFT_HAND_SIDE,
                           newInForInit: inForInit,
                           newAtStatementBegin: atStatementBegin);
     out(postfix.op);
   }
@@ skipping 681 matching lines @@
     declare(node.name);
     node.function.accept(this);
   }
 
   visitClassExpression(ClassExpression node) {
     declare(node.name);
     if (node.heritage != null) node.heritage.accept(this);
     for (Method element in node.methods) element.accept(this);
   }
 }