gdb: fix map prettyprinter

(gdb) p x
Python Exception <class 'gdb.error'> There is no member named b.:
$2 = map[string]string
->
(gdb) p x
$1 = map[string]string = {["shane"] = "hansen"}

Change-Id: I874d02a029f2ac9afc5ab666afb65760ec2c3177
Reviewed-on: https://go-review.googlesource.com/5522Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime-gdb.py

@@ -114,7 +114,7 @@ class MapTypePrinter:
 		return str(self.val.type)
 
 	def children(self):
-		B = self.val['b']
+		B = self.val['B']
 		buckets = self.val['buckets']
 		oldbuckets = self.val['oldbuckets']
 		flags = self.val['flags']

src/runtime/runtime-gdb_test.go

@@ -27,9 +27,15 @@ func checkGdbPython(t *testing.T) {
 const helloSource = `
 package main
 import "fmt"
-func main() {
+func finish() {
 	fmt.Println("hi")
 }
+func main() {
+	mapvar := make(map[string]string,5)
+	mapvar["abc"] = "def"
+	mapvar["ghi"] = "jkl"
+	finish()
+}
 `
 
 func TestGdbPython(t *testing.T) {
@@ -60,11 +66,15 @@ func TestGdbPython(t *testing.T) {
 
 	got, _ := exec.Command("gdb", "-nx", "-q", "--batch", "-iex",
 		fmt.Sprintf("add-auto-load-safe-path %s/src/runtime", runtime.GOROOT()),
-		"-ex", "br 'main.main'",
+		"-ex", "br 'main.finish'",
 		"-ex", "run",
+		"-ex", "up",
 		"-ex", "echo BEGIN info goroutines\n",
 		"-ex", "info goroutines",
 		"-ex", "echo END\n",
+		"-ex", "echo BEGIN print mapvar\n",
+		"-ex", "print mapvar",
+		"-ex", "echo END\n",
 		filepath.Join(dir, "a.exe")).CombinedOutput()
 
 	firstLine := bytes.SplitN(got, []byte("\n"), 2)[0]
@@ -83,4 +93,9 @@ func TestGdbPython(t *testing.T) {
 	if bl := blocks["info goroutines"]; !infoGoroutinesRe.MatchString(bl) {
 		t.Fatalf("info goroutines failed: %s", bl)
 	}
+
+	printMapvarRe := regexp.MustCompile(`\Q = map[string]string = {["abc"] = "def", ["ghi"] = "jkl"}\E$`)
+	if bl := blocks["print mapvar"]; !printMapvarRe.MatchString(bl) {
+		t.Fatalf("print mapvar failed: %s", bl)
+	}
 }