merge class extensions from patch files

merge class extensions from patch files.

There are a few examples of classes like this:

```dart
@patch
class StringBuffer {
  ... various @patch members ...
  // no @patch here
  void _writeString(str) { ... }
}
```

we need to merge in those members too

R=vsm@google.com

Committed: https://github.com/dart-lang/dev_compiler/commit/168f89313c6a6a4f2d5b111bdafc91cce14fd580

[Jennifer Messerly, 2015-02-25 23:50:35]

jmesserly@google.com changed reviewers:
+ vsm@google.com

[Jennifer Messerly, 2015-02-25 23:50:59]

this is based on my other CL that excludes @patch stuff from dart_codegen

[vsm, 2015-02-26 15:59:23]

lgtm

[Jennifer Messerly, 2015-02-26 16:08:25]

Committed patchset #1 (id:1) manually as
168f89313c6a6a4f2d5b111bdafc91cce14fd580 (presubmit successful).