Use metadata annotation @patch for patch classes

runtime/bin/common_patch.dart

Index: runtime/bin/common_patch.dart
diff --git a/runtime/bin/common_patch.dart b/runtime/bin/common_patch.dart
index feaf552696c25fa4afa0d2f7167e6168d534bafa..aa6e1aa6bb183f1cfe9257ee2385b08cf6c2e607 100644
--- a/runtime/bin/common_patch.dart
+++ b/runtime/bin/common_patch.dart
@@ -2,8 +2,8 @@
 // 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.
 
-patch class _IOCrypto {
-  /* patch */ static Uint8List getRandomBytes(int count)
+@patch class _IOCrypto {
+  /* @patch */ static Uint8List getRandomBytes(int count)

[Vyacheslav Egorov (Google), 2016/08/09 11:31:06]

I wonder if could make it 

@patch static Uint8List getRandomBytes(int count)

?

The motivation would be to completely match dart2js patch files (it annotates
both classes and members).

This has an additional benefit. Imagine situation where we have

core.dart

class Foo {
  external void _foo();
}

core_patch.dart

class Foo {
  @patch void _foo();
}

and then somebody removes or renames _foo in core.dart.

In this case if we have @patch annotation we can actually detect if the member
we are patching is actually there.

[hausner, 2016/08/09 15:48:45]

Yes, that would make sense. It's actually an old TODO from iposva.

Would it be fair to do this as a separate CL?

[Vyacheslav Egorov (Google), 2016/08/09 16:15:27]

SGTM!

       native "Crypto_GetRandomBytes";
 }