Add Unit() method to semantic tree

analyzer/tree.go

@@ -39,6 +39,11 @@ func (this *Tree) Analyze (
 	return this.analyzeDeclarations()
 }
 
+// Unit returns the root unit in this tree.
+func (this *Tree) Unit () uuid.UUID {
+	return this.unit
+}
+
 func (this *Tree) assembleRawMaps () error {
 	for _, declaration := range this.ast.Declarations {
 		switch declaration := declaration.(type) {