cmd/cover: show file coverage in HTML drop down

As requested on Stack Overflow: http://goo.gl/ams9fY
(Kudos to sberry for his JavaScript solution, provided there.
This change does the same thing on the server side.)

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/127030043

cmd/cover/html.go

@@ -50,8 +50,9 @@ func htmlOutput(profile, outfile string) error {
 			return err
 		}
 		d.Files = append(d.Files, &templateFile{
-			Name: fn,
-			Body: template.HTML(buf.String()),
+			Name:     fn,
+			Body:     template.HTML(buf.String()),
+			Coverage: percentCovered(profile),
 		})
 	}
 
@@ -83,6 +84,23 @@ func htmlOutput(profile, outfile string) error {
 	return nil
 }
 
+// percentCovered returns, as a percentage, the fraction of the statements in
+// the profile covered by the test run.
+// In effect, it reports the coverage of a given source file.
+func percentCovered(p *cover.Profile) float64 {
+	var total, covered int64
+	for _, b := range p.Blocks {
+		total += int64(b.NumStmt)
+		if b.Count > 0 {
+			covered += int64(b.NumStmt)
+		}
+	}
+	if total == 0 {
+		return 0
+	}
+	return float64(covered) / float64(total) * 100
+}
+
 // htmlGen generates an HTML coverage report with the provided filename,
 // source code, and tokens, and writes it to the given Writer.
 func htmlGen(w io.Writer, src []byte, boundaries []cover.Boundary) error {
@@ -166,8 +184,9 @@ type templateData struct {
 }
 
 type templateFile struct {
-	Name string
-	Body template.HTML
+	Name     string
+	Body     template.HTML
+	Coverage float64
 }
 
 const tmplHTML = `
@@ -215,7 +234,7 @@ const tmplHTML = `
 			<div id="nav">
 				<select id="files">
 				{{range $i, $f := .Files}}
-				<option value="file{{$i}}">{{$f.Name}}</option>
+				<option value="file{{$i}}">{{$f.Name}} ({{printf "%.1f" $f.Coverage}}%)</option>
 				{{end}}
 				</select>
 			</div>