go.tools/ssa/interp: fix windows build.

- Add missing import.
- platform-specific files syswrite() wrapper presents
  standard API to syscall.Write, even on Windows where
  first arg is a Handle.

Fixes golang/go#7100

R=gri, minux.ma, alex.brainman
CC=golang-codereviews
https://golang.org/cl/52250043

ssa/interp/external_plan9.go

@@ -5,6 +5,8 @@
 package interp
 
 import (
+	"syscall"
+
 	"code.google.com/p/go.tools/ssa"
 )
 
@@ -43,3 +45,7 @@ func ext۰syscall۰Write(fn *ssa.Function, args []value) value {
 func ext۰syscall۰RawSyscall(fn *ssa.Function, args []value) value {
 	return tuple{^uintptr(0), uintptr(0), uintptr(0)}
 }
+
+func syswrite(fd int, b []byte) (int, error) {
+	return syscall.Write(fd, b)
+}

ssa/interp/external_unix.go

@@ -130,3 +130,7 @@ func ext۰syscall۰Write(fn *ssa.Function, args []value) value {
 func ext۰syscall۰RawSyscall(fn *ssa.Function, args []value) value {
 	return tuple{uintptr(0), uintptr(0), uintptr(syscall.ENOSYS)}
 }
+
+func syswrite(fd int, b []byte) (int, error) {
+	return syscall.Write(fd, b)
+}

ssa/interp/external_windows.go

@@ -5,6 +5,8 @@
 package interp
 
 import (
+	"syscall"
+
 	"code.google.com/p/go.tools/ssa"
 )
 
@@ -41,3 +43,6 @@ func ext۰syscall۰Write(fn *ssa.Function, args []value) value {
 func ext۰syscall۰RawSyscall(fn *ssa.Function, args []value) value {
 	return tuple{uintptr(0), uintptr(0), uintptr(syscall.ENOSYS)}
 }
+func syswrite(fd int, b []byte) (int, error) {
+	panic("syswrite not yet implemented")
+}

ssa/interp/ops.go

@@ -10,7 +10,6 @@ import (
 	"go/token"
 	"strings"
 	"sync"
-	"syscall"
 	"unsafe"
 
 	"code.google.com/p/go.tools/go/exact"
@@ -910,12 +909,13 @@ var capturedOutputMu sync.Mutex
 // The print/println built-ins and the write() system call funnel
 // through here so they can be captured by the test driver.
 func write(fd int, b []byte) (int, error) {
+	// TODO(adonovan): fix: on Windows, std{out,err} are not 1, 2.
 	if CapturedOutput != nil && (fd == 1 || fd == 2) {
 		capturedOutputMu.Lock()
 		CapturedOutput.Write(b) // ignore errors
 		capturedOutputMu.Unlock()
 	}
-	return syscall.Write(fd, b)
+	return syswrite(fd, b)
 }
 
 // callBuiltin interprets a call to builtin fn with arguments args,