[CFE] Speedup dill serialization after slowdown in 548bff1

548bff1 was meant to do less.
It actually turns out slower though:

Before 548bff1:
[ 07:52 | 100.0% | + 2252 | -    0 ]: weak/variance/unconstrained_inference

real    7m58.017s
user    11m15.944s
sys     2m42.890s

=> 209.5 ms per test.

After 548bff1:
[ 08:47 | 100.0% | + 2252 | -    0 ]: weak/variance/unconstrained_inference

real    8m53.362s
user    10m27.086s
sys     1m10.925s

=> 234.0 ms per test.

What it did was, that it introduced - I'm guessing this is the reason -
some non-monomorphism in the serialization code by having sinks of
different types.

Running
`time out/ReleaseX64/dart --enable-asserts pkg/front_end/test/fasta/weak_suite.dart --traceStepTiming -DupdateExpectations=false -DsemiFuzz=false`
(with code changed so it's three shards, running only the first one)

I get numbers like this for writing the dill file though:

Before 548bff1: 0:01:28.369107
 After 548bff1: 0:01:44.803383

(again this is for 1 out of 3 shards, but illustrates the point ---
serialization is slower).

This CL changes this to always serialize into the same sink (in memory),
then - if needed (as in the original CL) - write it to disk.
This gets (again with 1 out of 3 shards) writing the dill down to
0:01:21.872812, and the total run looks like this:

[ 07:18 | 100.0% | + 2252 | -    0 ]: weak/variance/unconstrained_inference

real    7m23.361s
user    10m48.069s
sys     1m30.336s

=> 194.4 ms per test.

(rebased run:

[ 07:13 | 100.0% | + 2254 | -    0 ]: weak/variance/unconstrained_inference

real    7m18.828s
user    10m44.163s
sys     1m31.777s

=> 192.1 ms per test.)

Change-Id: I2906807bbab407fb499aa910afd20f41347bc3c6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/245374
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

pkg/front_end/test/utils/kernel_chain.dart

@@ -7,8 +7,6 @@ library fasta.testing.kernel_chain;
 import 'dart:async';
 import 'dart:io' show Directory, File, IOSink, Platform;
 
-import 'dart:typed_data' show Uint8List;
-
 import 'package:_fe_analyzer_shared/src/util/relativize.dart'
     show isWindows, relativizeUri;
 
@@ -454,51 +452,38 @@ class WriteDill extends Step<ComponentResult, ComponentResult, ChainContext> {
         writeToFile = false;
       }
     }
-
-    Sink<List<int>> sink;
-    String writeMessage;
-    if (writeToFile && !skipVm) {
-      Directory tmp = await Directory.systemTemp.createTemp();
-      Uri uri = tmp.uri.resolve("generated.dill");
-      File generated = new File.fromUri(uri);
-      sink = generated.openWrite();
-      result = new ComponentResult(
-          result.description,
-          result.component,
-          result.userLibraries,
-          result.compilationSetup,
-          result.sourceTarget,
-          uri);
-      writeMessage = "Wrote component to `${generated.path}`";
-    } else {
-      sink = new DevNullSink();
-      writeMessage = "Wrote component to /dev/null";
-    }
+    ByteSink sink = new ByteSink();
+    bool good = false;
     try {
       // TODO(johnniwinther,jensj): Avoid serializing the sdk.
       new BinaryPrinter(sink).writeComponentFile(component);
+      good = true;
     } catch (e, s) {
       return fail(result, e, s);
     } finally {
-      print(writeMessage);
-      if (sink is IOSink) {
-        await sink.close();
+      if (good && writeToFile && !skipVm) {
+        Directory tmp = await Directory.systemTemp.createTemp();
+        Uri uri = tmp.uri.resolve("generated.dill");
+        File generated = new File.fromUri(uri);
+        IOSink ioSink = generated.openWrite();
+        ioSink.add(sink.builder.takeBytes());
+        await ioSink.close();
+        result = new ComponentResult(
+            result.description,
+            result.component,
+            result.userLibraries,
+            result.compilationSetup,
+            result.sourceTarget,
+            uri);
+        print("Wrote component to `${generated.path}`.");
       } else {
-        sink.close();
+        print("Wrote component to memory.");
       }
     }
     return pass(result);
   }
 }
 
-class DevNullSink<T> extends Sink<T> {
-  @override
-  void add(T data) {}
-
-  @override
-  void close() {}
-}
-
 class ReadDill extends Step<Uri, Uri, ChainContext> {
   const ReadDill();
 
@@ -516,32 +501,6 @@ class ReadDill extends Step<Uri, Uri, ChainContext> {
   }
 }
 
-class BytesCollector implements Sink<List<int>> {
-  final List<List<int>> lists = <List<int>>[];
-
-  int length = 0;
-
-  @override
-  void add(List<int> data) {
-    lists.add(data);
-    length += data.length;
-  }
-
-  Uint8List collect() {
-    Uint8List result = new Uint8List(length);
-    int offset = 0;
-    for (List<int> list in lists) {
-      result.setRange(offset, offset += list.length, list);
-    }
-    lists.clear();
-    length = 0;
-    return result;
-  }
-
-  @override
-  void close() {}
-}
-
 Future<String> runDiff(Uri expected, String actual) async {
   if (Platform.isWindows) {
     // TODO(johnniwinther): Work-around for Windows. For some reason piping