Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/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 199 matching lines @@
           // TODO(ahe): Remove special case for negate.
           write(' ');
         }
       }
       visit(send.selector);
     } else {
       visit(name);
     }
   }
 
+  visitAsyncModifier(AsyncModifier node) {
+    write(node.asyncToken.value);
+    if (node.starToken != null) {
+      write(node.starToken.value);
+    }
+  }
+
   visitFunctionExpression(FunctionExpression node) {
     if (!node.modifiers.nodes.isEmpty) {
       visit(node.modifiers);
       write(' ');
     }
     if (node.returnType != null && !stripTypes) {
       visit(node.returnType);
       write(' ');
     }
     if (node.getOrSet != null) {
       write(node.getOrSet.value);
       write(' ');
     }
     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();
     visit(node.body);
   }
 
   visitIdentifier(Identifier node) {
     write(node.token.value);
   }
 
   visitIf(If node) {
     write(node.ifToken.value);
@@ skipping 119 matching lines @@
   visitReturn(Return node) {
     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);
+    space();
+    visit(node.expression);
+    write(node.endToken);
+  }
+
+
   unparseSendReceiver(Send node, {bool spacesNeeded: false}) {
     if (node.receiver == null) return;
     visit(node.receiver);
     CascadeReceiver asCascadeReceiver = node.receiver.asCascadeReceiver();
     if (asCascadeReceiver != null) {
       newline();
       indentMore();
       indentMore();
       write(asCascadeReceiver.cascadeOperator.value);
       indentLess();
@@ skipping 90 matching lines @@
       visit(node.argumentsNode);
     }
   }
 
   visitThrow(Throw node) {
     write(node.throwToken.value);
     write(' ');
     visit(node.expression);
   }
 
+  visitAwait(Await node) {
+    write(node.awaitToken.value);
+    write(' ');
+    visit(node.expression);
+  }
+
   visitTypeAnnotation(TypeAnnotation node) {
     visit(node.typeName);
     visit(node.typeArguments);
   }
 
   visitTypeVariable(TypeVariable node) {
     visit(node.name);
     if (node.bound != null) {
       write(' extends ');
       visit(node.bound);
@@ skipping 74 matching lines @@
 
   visitBreakStatement(BreakStatement node) {
     visitGotoStatement(node);
   }
 
   visitContinueStatement(ContinueStatement node) {
     visitGotoStatement(node);
   }
 
   visitForIn(ForIn node) {
+    if (node.awaitToken != null) {
+      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);
@@ skipping 199 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }