Add a CompilerPool class.

lib/src/runner/browser/compiler_pool.dart

Index: lib/src/runner/browser/compiler_pool.dart
diff --git a/lib/src/runner/browser/compiler_pool.dart b/lib/src/runner/browser/compiler_pool.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cf77fe06ed553ba04f020fe2b8a707dc9caf2a4f
--- /dev/null
+++ b/lib/src/runner/browser/compiler_pool.dart
@@ -0,0 +1,134 @@
+// 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.
+
+library unittest.util.compiler_pool;
+
+import 'dart:async';
+import 'dart:collection';
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+import 'package:pool/pool.dart';
+
+import '../../util/io.dart';
+import '../load_exception.dart';
+
+/// A pool of `dart2js` instances.
+///
+/// This limits the number of compiler instances running concurrently. It also
+/// ensures that their output doesn't intermingle; only one instance is
+/// "visible" (that is, having its output printed) at a time, and the other
+/// instances' output is buffered until it's their turn to be visible.
+class CompilerPool {
+  /// The internal pool that controls the number of process running at once.
+  final Pool _pool;
+
+  /// The currently-active compilers.
+  ///
+  /// The first one is the only visible the compiler; the rest will become
+  /// visible in queue order. Note that some of these processes may actually
+  /// have already exited; they're kept around so that their output can be
+  /// emitted once they become visible.
+  final _compilers = new Queue<_Compiler>();
+
+  /// Creates a compiler pool that runs up to [parallel] instances of `dart2js`
+  /// at once.
+  ///
+  /// If [parallel] isn't provided, it defaults to 4.
+  CompilerPool({int parallel})
+      : _pool = new Pool(parallel == null ? 4 : parallel);
+
+  /// Compile the Dart code at [dartPath] to [jsPath].
+  ///
+  /// This wraps the Dart code in the standard browser-testing wrapper. If
+  /// [packageRoot] is provided, it's used as the package root for the
+  /// compilation.
+  ///
+  /// The returned [Future] will complete once the `dart2js` process completes
+  /// *and* all its output has been printed to the command line.
+  Future compile(String dartPath, String jsPath, {String packageRoot}) {
+    return _pool.withResource(() {
+      return withTempDir((dir) {
+        var wrapperPath = p.join(dir, "runInBrowser.dart");
+        new File(wrapperPath).writeAsStringSync('''
+import "package:unittest/src/runner/browser/iframe_listener.dart";
+
+import "${p.toUri(p.absolute(dartPath))}" as test;
+
+void main(_) {
+  IframeListener.start(() => test.main);
+}
+''');
+
+        var dart2jsPath = p.join(sdkDir, 'bin', 'dart2js');
+        if (Platform.isWindows) dart2jsPath += '.bat';
+
+        var args = ["--checked", wrapperPath, "--out=$jsPath"];
+
+        if (packageRoot != null) {
+          args.add("--package-root=${p.absolute(packageRoot)}");
+        }
+
+        if (canUseSpecialChars) {
+          args.add("--enable-diagnostic-colors");
+        }
+
+        return Process.start(dart2jsPath, args).then((process) {
+          var compiler = new _Compiler(dartPath, process);
+
+          if (_compilers.isEmpty) _showProcess(compiler);
+          _compilers.add(compiler);
+
+          return compiler.onDone;
+        });
+      });
+    });
+  }
+
+  /// Mark [compiler] as the visible instance.
+  ///
+  /// This prints all [compiler]'s standard output and error.
+  void _showProcess(_Compiler compiler) {
+    print("Compiling ${compiler.path}...");
+
+    // We wait for stdout and stderr to close and for exitCode to fire to ensure
+    // that we're done printing everything about one process before we start the
+    // next.
+    Future.wait([
+      compiler.process.stdout.listen(stdout.add).asFuture(),
+      compiler.process.stderr.listen(stderr.add).asFuture(),
+      compiler.process.exitCode.then((exitCode) {
+        if (exitCode == 0) return;
+        throw new LoadException(compiler.path, "dart2js failed.");
+      })
+    ]).then(compiler.onDoneCompleter.complete)
+        .catchError(compiler.onDoneCompleter.completeError)
+        .then((_) {
+      _compilers.removeFirst();
+      if (_compilers.isEmpty) return;
+
+      var next = _compilers.first;
+
+      // Wait a bit before printing the next progress in case the current one
+      // threw an error that needs to be printed.
+      Timer.run(() => _showProcess(next));
+    });
+  }
+}
+
+/// A running instance of `dart2js`.
+class _Compiler {
+  /// The path of the Dart file being compiled.
+  final String path;
+
+  /// The underlying process.
+  final Process process;
+
+  /// A future that will complete once this instance has finished running and
+  /// all its output has been printed.
+  Future get onDone => onDoneCompleter.future;
+  final onDoneCompleter = new Completer();
+
+  _Compiler(this.path, this.process);
+}