Model 'await for' as an explicit node in tree and fix its handling.

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 605 matching lines @@
   }
 
   visitBreakStatement(BreakStatement node) {
     visitGotoStatement(node);
   }
 
   visitContinueStatement(ContinueStatement node) {
     visitGotoStatement(node);
   }
 
-  visitForIn(ForIn node) {
-    if (node.awaitToken != null) {
-      write(node.awaitToken.value);
-      write(' ');
-    }
+  visitAsyncForIn(AsyncForIn node) {
+    write(node.awaitToken.value);
+    write(' ');
     write(node.forToken.value);
     space();
     write('(');
+    visit(node.declaredIdentifier);
+    write(' ');
+    addToken(node.inToken);
+    visit(node.expression);
+    write(')');
+    space();
+    visit(node.body);
+  }
+
+
+  visitSyncForIn(SyncForIn node) {
+    write(node.forToken.value);
+    space();
+    write('(');
     visit(node.declaredIdentifier);
     write(' ');
     addToken(node.inToken);
     visit(node.expression);
     write(')');
     space();
     visit(node.body);
   }
 
   visitLabel(Label node) {
@@ skipping 196 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }