Support async/await in dart2dart.

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 235 matching lines @@
     }
     unparseFunctionName(node.name);
     visit(node.parameters);
     if (node.initializers != null) {
       space();
       write(':');
       space();
       unparseNodeListFrom(node.initializers, node.initializers.nodes,
           spaces: true);
     }
-    visit(node.asyncModifier);
-    if (node.body != null && node.body is! EmptyStatement) space();
+    if (node.asyncModifier != null) {
+      if (node.getOrSet != null) {
+        write(' ');
+      } else {
+        // Space is optional if this is not a getter.
+        space();
+      }
+      visit(node.asyncModifier);
+    }
+    if (node.body != null && node.body is! EmptyStatement) {
+      space();
+    }
     visit(node.body);
   }
 
   visitIdentifier(Identifier node) {
     write(node.token.value);
   }
 
   visitIf(If node) {
     write(node.ifToken.value);
     space();
@@ skipping 119 matching lines @@
     write(node.beginToken.value);
     if (node.hasExpression && node.beginToken.stringValue != '=>') {
       write(' ');
     }
     if (node.beginToken.stringValue == '=>') space();
     visit(node.expression);
     if (node.endToken != null) write(node.endToken.value);
   }
 
   visitYield(Yield node) {
-    write(node.yieldToken);
-    write(node.starToken);
+    write(node.yieldToken.value);
+    if (node.starToken != null) {
+      write(node.starToken.value);
+    }
     space();
     visit(node.expression);
-    write(node.endToken);
+    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) {
       newline();
       indentMore();
@@ skipping 416 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }