Fix various nits in VM patch files.

runtime/lib/async_patch.dart

 // Copyright (c) 2015, 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:_internal";
+import "dart:_internal" hide Symbol;
 
 // We need to pass the value as first argument and leave the second and third
 // arguments empty (used for error handling).
 // See vm/ast_transformer.cc for usage.
 Function _asyncThenWrapperHelper(continuation) {
   // Any function that is used as an asynchronous callback must be registered
   // in the current Zone. Normally, this is done by the future when a
   // callback is registered (for example with `.then` or `.catchError`). In our
   // case we want to reuse the same callback multiple times and therefore avoid
   // the multiple registrations. For our internal futures (`_Future`) we can
@@ skipping 178 matching lines @@
 
 /// Returns a [StackTrace] object containing the synchronous prefix for this
 /// asynchronous method.
 Object _asyncStackTraceHelper() native "StackTrace_asyncStackTraceHelper";
 
 void _clearAsyncThreadStackTrace()
     native "StackTrace_clearAsyncThreadStackTrace";
 
 void _setAsyncThreadStackTrace(StackTrace stackTrace) native
     "StackTrace_setAsyncThreadStackTrace";