Create abstraction layer for dart:io's Process commands (#7100)

With this change, they're run via instance methods on an object
obtained through the context. This will allow us to substitute
that object in tests with replay/record versions to allow us to
mock out the os-layer in tests.

packages/flutter_tools/lib/src/base/process_manager.dart

diff --git a/packages/flutter_tools/lib/src/base/process_manager.dart b/packages/flutter_tools/lib/src/base/process_manager.dart
new file mode 100644
index 0000000..0cfc1f5
--- /dev/null
+++ b/packages/flutter_tools/lib/src/base/process_manager.dart
@@ -0,0 +1,66 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'context.dart';
+
+ProcessManager get processManager => context[ProcessManager];
+
+class ProcessManager {
+  Future<Process> start(
+      String executable,
+      List<String> arguments,
+      {String workingDirectory,
+       Map<String, String> environment,
+       ProcessStartMode mode: ProcessStartMode.NORMAL}) {
+    return Process.start(
+      executable,
+      arguments,
+      workingDirectory: workingDirectory,
+      environment: environment,
+      mode: mode,
+    );
+  }
+
+  Future<ProcessResult> run(
+      String executable,
+      List<String> arguments,
+      {String workingDirectory,
+       Map<String, String> environment,
+       Encoding stdoutEncoding: SYSTEM_ENCODING,
+       Encoding stderrEncoding: SYSTEM_ENCODING}) {
+    return Process.run(
+      executable,
+      arguments,
+      workingDirectory: workingDirectory,
+      environment: environment,
+      stdoutEncoding: stdoutEncoding,
+      stderrEncoding: stderrEncoding,
+    );
+  }
+
+  ProcessResult runSync(
+      String executable,
+      List<String> arguments,
+      {String workingDirectory,
+       Map<String, String> environment,
+       Encoding stdoutEncoding: SYSTEM_ENCODING,
+       Encoding stderrEncoding: SYSTEM_ENCODING}) {
+    return Process.runSync(
+      executable,
+      arguments,
+      workingDirectory: workingDirectory,
+      environment: environment,
+      stdoutEncoding: stdoutEncoding,
+      stderrEncoding: stderrEncoding,
+    );
+  }
+
+  bool killPid(int pid, [ProcessSignal signal = ProcessSignal.SIGTERM]) {
+    return Process.killPid(pid, signal);
+  }
+}