cmd/internal/testing/stdlibsymbol: fix build error

The type for frontend.Symbol has changed since tests passed for
CL 300173, which caused build errors that are fixed.

Change-Id: I71264a14dddaf9ef7a417091db9455bd9b9455ee
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/304089
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

cmd/internal/testing/stdlibsymbol/main.go

@@ -93,12 +93,14 @@ func compareVersionsPage(path string) error {
 		for _, vs := range vl.Versions {
 			v := stdlib.VersionForTag(vs.Version)
 			versionToNameToSymbol[v] = map[string]*internal.UnitSymbol{}
-			for _, s := range vs.Symbols {
-				if s.New {
-					versionToNameToSymbol[v][s.Name] = unitSymbol(s)
-				}
-				for _, c := range s.Children {
-					versionToNameToSymbol[v][c.Name] = unitSymbol(c)
+			for _, syms := range vs.Symbols {
+				for _, s := range syms {
+					if s.New {
+						versionToNameToSymbol[v][s.Name] = unitSymbol(s)
+					}
+					for _, c := range s.Children {
+						versionToNameToSymbol[v][c.Name] = unitSymbol(c)
+					}
 				}
 			}
 		}