Implement await and async for the js-ast.

tests/compiler/dart2js/js_parser_test.dart

 // Copyright (c) 2011, 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.
 
 import 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'mock_compiler.dart';
 import 'package:compiler/src/js/js.dart' as jsAst;
 import 'package:compiler/src/js/js.dart' show js;
@@ skipping 119 matching lines @@
     testExpression('!x == false'),
     testExpression('var foo = void 0'),
     testExpression('delete foo.bar'),
     testExpression('delete foo'),
     testExpression('x in y'),
     testExpression('x instanceof y'),
     testExpression('a * b in c * d'),
     testExpression('a * b instanceof c * d'),
     testError('x typeof y', 'Unparsed junk'),
     testExpression('x &= ~mask'),
+    // Await is parsed as an unary prefix operator.
+    testExpression('var foo = await 0'),
+    testExpression('await x++'),
+    testExpression('void (await (x++))', 'void await x++'),
+    testExpression('void (await x)++'),
+    testExpression('++(await x)++'),
     // Adjacent tokens.
     testExpression('foo[x[bar]]'),
     testExpression('foo[[bar]]'),
     // Prefix ++ etc.
     testExpression("++x"),
     testExpression("++foo.bar"),
     testExpression("+x"),
     testExpression("+foo.bar"),
     testExpression("-x"),
     testExpression("-foo.bar"),
@@ skipping 37 matching lines @@
     testExpression("y = a == null ? b + c : a + c"),
     testExpression("foo = a ? b : c ? d : e"),
     testExpression("foo = a ? b ? c : d : e"),
     testExpression("foo = (a = v) ? b = w : c = x ? d = y : e = z"),
     testExpression("foo = (a = v) ? b = w ? c = x : d = y : e = z"),
     // Stacked assignment.
     testExpression("a = b = c"),
     testExpression("var a = b = c"),
   ]));
 }