Remove terminal.Writer

terminal.Writer is now just a wrapper around stdio.Stdout() without
any useful functionality.  Replace it with stdio.Stdout() as an
io.Writer.

Test: ui/terminal/status_test.go
Change-Id: I5bc5476afdca950b505642f0135a3af9d37fbe24

ui/terminal/stdio.go

@@ -0,0 +1,55 @@
+// Copyright 2018 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package terminal provides a set of interfaces that can be used to interact
+// with the terminal (including falling back when the terminal is detected to
+// be a redirect or other dumb terminal)
+package terminal
+
+import (
+	"io"
+	"os"
+)
+
+// StdioInterface represents a set of stdin/stdout/stderr Reader/Writers
+type StdioInterface interface {
+	Stdin() io.Reader
+	Stdout() io.Writer
+	Stderr() io.Writer
+}
+
+// StdioImpl uses the OS stdin/stdout/stderr to implement StdioInterface
+type StdioImpl struct{}
+
+func (StdioImpl) Stdin() io.Reader  { return os.Stdin }
+func (StdioImpl) Stdout() io.Writer { return os.Stdout }
+func (StdioImpl) Stderr() io.Writer { return os.Stderr }
+
+var _ StdioInterface = StdioImpl{}
+
+type customStdio struct {
+	stdin  io.Reader
+	stdout io.Writer
+	stderr io.Writer
+}
+
+func NewCustomStdio(stdin io.Reader, stdout, stderr io.Writer) StdioInterface {
+	return customStdio{stdin, stdout, stderr}
+}
+
+func (c customStdio) Stdin() io.Reader  { return c.stdin }
+func (c customStdio) Stdout() io.Writer { return c.stdout }
+func (c customStdio) Stderr() io.Writer { return c.stderr }
+
+var _ StdioInterface = customStdio{}