Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/tree/prettyprint.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;
 
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
@@ skipping 103 matching lines @@
     node.accept(p);
     return p.sb.toString();
   }
 
   visitNodeWithChildren(Node node, String type) {
     openNode(node, type);
     node.visitChildren(this);
     closeNode();
   }
 
+  visitAsyncModifier(AsyncModifier node) {
+    openAndCloseNode(node, "AsyncModifier",
+        {'asyncToken': node.asyncToken,
+         'starToken': node.starToken});
+  }
+
   visitBlock(Block node) {
     visitNodeWithChildren(node, "Block");
   }
 
   visitBreakStatement(BreakStatement node) {
     visitNodeWithChildren(node, "BreakStatement");
   }
 
   visitCascade(Cascade node) {
     visitNodeWithChildren(node, "Cascade");
@@ skipping 40 matching lines @@
 
   visitExpressionStatement(ExpressionStatement node) {
     visitNodeWithChildren(node, "ExpressionStatement");
   }
 
   visitFor(For node) {
     visitNodeWithChildren(node, "For");
   }
 
   visitForIn(ForIn node) {
-    visitNodeWithChildren(node, "ForIn");
+    openNode(node, "ForIn", {'await': node.awaitToken});
+    node.visitChildren(this);
+    closeNode();
   }
 
   visitFunctionDeclaration(FunctionDeclaration node) {
     visitNodeWithChildren(node, "FunctionDeclaration");
   }
 
   visitFunctionExpression(FunctionExpression node) {
     openNode(node, "FunctionExpression", {
       "getOrSet" : tokenToStringOrNull(node.getOrSet)
     });
@@ skipping 116 matching lines @@
   visitRethrow(Rethrow node) {
     visitNodeWithChildren(node, "Rethrow");
   }
 
   visitReturn(Return node) {
     openNode(node, "Return");
     visitChildNode(node.expression, "expression");
     closeNode();
   }
 
+  visitYield(Yield node) {
+    openNode(node, "Yield", {'star': node.starToken});
+    visitChildNode(node.expression, "expression");
+    closeNode();
+  }
+
   visitChildNode(Node node, String fieldName) {
     if (node == null) return;
     addCurrentIndent();
     sb.write("<$fieldName>\n");
     pushTag(fieldName);
     node.accept(this);
     popTag();
     addCurrentIndent();
     sb.write("</$fieldName>\n");
   }
@@ skipping 43 matching lines @@
   visitLiteralSymbol(LiteralSymbol node) {
     openNode(node, "LiteralSymbol");
     visitChildNode(node.identifiers, "identifiers");
     closeNode();
   }
 
   visitThrow(Throw node) {
     visitNodeWithChildren(node, "Throw");
   }
 
+  visitAwait(Await node) {
+    visitNodeWithChildren(node, "Await");
+  }
+
   visitTryStatement(TryStatement node) {
     visitNodeWithChildren(node, "TryStatement");
   }
 
   visitTypeAnnotation(TypeAnnotation node) {
     openNode(node, "TypeAnnotation");
     visitChildNode(node.typeName, "typeName");
     visitChildNode(node.typeArguments, "typeArguments");
     closeNode();
   }
@@ skipping 112 matching lines @@
   }
 
   visitGotoStatement(GotoStatement node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }