Fix a batch of DDC SDK compile errors.

sdk/lib/io/file_impl.dart

Index: sdk/lib/io/file_impl.dart
diff --git a/sdk/lib/io/file_impl.dart b/sdk/lib/io/file_impl.dart
index fd66fc1b6094de886ce9e268d819bb502b783fa3..fe8a8bfbff80a69ebb57f326641a1f1e01e9dbef 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -198,8 +198,7 @@ class _FileStreamConsumer extends StreamConsumer<List<int>> {
     return completer.future;
   }
 
-  Future<File> close() =>
-      _openFuture.then<File>((openedFile) => openedFile.close());
+  Future close() => _openFuture.then((openedFile) => openedFile.close());

[floitsch, 2017/04/27 22:10:25]

Please use my changes instead:
https://codereview.chromium.org/2764943002/

[Bob Nystrom, 2017/04/28 00:13:07]

Done. (FWIW, I changed it the way I did because _FileStream is private and every
use of its close() method I saw didn't use the result value anyway.)

 }
 
 // Class for encapsulating the native implementation of files.
@@ -555,11 +554,14 @@ class _File extends FileSystemEntity implements File {
 
   Future<File> writeAsBytes(List<int> bytes,
       {FileMode mode: FileMode.WRITE, bool flush: false}) {
-    return open(mode: mode).then((file) {
-      return file.writeFrom(bytes, 0, bytes.length).then((_) {
-        if (flush) return file.flush().then((_) => this);
-        return this;
-      }).whenComplete(file.close);
+    return open(mode: mode).then((file) async {

[floitsch, 2017/04/27 22:10:25]

The core libraries currently do not use 'async'. I wouldn't start here.
Especially, since there is no need.

[Bob Nystrom, 2017/04/28 00:13:07]

Took your change instead. I could have sworn I tried that same fix but it didn't
fully satisfy the type system, I think related to the body of the function
returning a Future<File> in one branch and a File in the other. Seems to be
working now. <shrug> Maybe I put the <File> on the wrong .then().

Is there a reason the core libraries don't use async? I'd expect us to want to
always dogfood the latest language features when we can.

+      try {
+        await file.writeFrom(bytes, 0, bytes.length);
+        if (flush) await file.flush();
+        return this as File;
+      } finally {
+        await file.close();
+      }
     });
   }