runtime: use strings.Builder

Change-Id: I9274d36ca983fdf59088b71a97b139ea262370dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/428276
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/align_test.go

@@ -5,7 +5,6 @@
 package runtime_test
 
 import (
-	"bytes"
 	"go/ast"
 	"go/build"
 	"go/importer"
@@ -180,7 +179,7 @@ func (v *Visitor) checkAddr(n ast.Node) {
 }
 
 func (v *Visitor) print(n ast.Node) string {
-	var b bytes.Buffer
+	var b strings.Builder
 	printer.Fprint(&b, v.fset, n)
 	return b.String()
 }

src/runtime/debuglog_test.go

@@ -23,7 +23,6 @@
 package runtime_test
 
 import (
-	"bytes"
 	"fmt"
 	"regexp"
 	"runtime"
@@ -94,7 +93,7 @@ func TestDebugLogInterleaving(t *testing.T) {
 		}
 		wg.Done()
 	}()
-	var want bytes.Buffer
+	var want strings.Builder
 	for i := 0; i < 1000; i++ {
 		runtime.Dlog().I(i).End()
 		fmt.Fprintf(&want, "[] %d\n", i)
@@ -122,7 +121,7 @@ func TestDebugLogWraparound(t *testing.T) {
 
 	runtime.ResetDebugLog()
 	var longString = strings.Repeat("a", 128)
-	var want bytes.Buffer
+	var want strings.Builder
 	for i, j := 0, 0; j < 2*runtime.DebugLogBytes; i, j = i+1, j+len(longString) {
 		runtime.Dlog().I(i).S(longString).End()
 		fmt.Fprintf(&want, "[] %d %s\n", i, longString)

src/runtime/signal_windows_test.go

@@ -93,8 +93,8 @@ func TestCtrlHandler(t *testing.T) {
 
 	// run test program
 	cmd = exec.Command(exe)
-	var stdout bytes.Buffer
-	var stderr bytes.Buffer
+	var stdout strings.Builder
+	var stderr strings.Builder
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr
 	inPipe, err := cmd.StdinPipe()

src/runtime/stack_test.go

@@ -5,7 +5,6 @@
 package runtime_test
 
 import (
-	"bytes"
 	"fmt"
 	"reflect"
 	"regexp"
@@ -778,7 +777,7 @@ func TestTracebackSystemstack(t *testing.T) {
 	// and that we see TestTracebackSystemstack.
 	countIn, countOut := 0, 0
 	frames := CallersFrames(pcs)
-	var tb bytes.Buffer
+	var tb strings.Builder
 	for {
 		frame, more := frames.Next()
 		fmt.Fprintf(&tb, "\n%s+0x%x %s:%d", frame.Function, frame.PC-frame.Entry, frame.File, frame.Line)