Add Dart Sass support

But note that the Dart Sass Embedded Protocol is still in beta (beta 5), a main release scheduled for Q1 2021.

Fixes #7380
Fixes #8102

deps/deps.go

@@ -94,6 +94,9 @@ type Deps struct {
 	// BuildStartListeners will be notified before a build starts.
 	BuildStartListeners *Listeners
 
+	// Resources that gets closed when the build is done or the server shuts down.
+	BuildClosers *Closers
+
 	// Atomic values set during a build.
 	// This is common/global for all sites.
 	BuildState *BuildState
@@ -284,6 +287,7 @@ func New(cfg DepsCfg) (*Deps, error) {
 		Site:                    cfg.Site,
 		FileCaches:              fileCaches,
 		BuildStartListeners:     &Listeners{},
+		BuildClosers:            &Closers{},
 		BuildState:              buildState,
 		Running:                 cfg.Running,
 		Timeout:                 time.Duration(timeoutms) * time.Millisecond,
@@ -297,6 +301,10 @@ func New(cfg DepsCfg) (*Deps, error) {
 	return d, nil
 }
 
+func (d *Deps) Close() error {
+	return d.BuildClosers.Close()
+}
+
 // ForLanguage creates a copy of the Deps with the language dependent
 // parts switched out.
 func (d Deps) ForLanguage(cfg DepsCfg, onCreated func(d *Deps) error) (*Deps, error) {
@@ -399,3 +407,30 @@ func (b *BuildState) Incr() int {
 func NewBuildState() BuildState {
 	return BuildState{}
 }
+
+type Closer interface {
+	Close() error
+}
+
+type Closers struct {
+	mu sync.Mutex
+	cs []Closer
+}
+
+func (cs *Closers) Add(c Closer) {
+	cs.mu.Lock()
+	defer cs.mu.Unlock()
+	cs.cs = append(cs.cs, c)
+}
+
+func (cs *Closers) Close() error {
+	cs.mu.Lock()
+	defer cs.mu.Unlock()
+	for _, c := range cs.cs {
+		c.Close()
+	}
+
+	cs.cs = cs.cs[:0]
+
+	return nil
+}