Use single map of Source to modification time.

pkg/analyzer/tool/task_dependency_graph/tasks.dot

Index: pkg/analyzer/tool/task_dependency_graph/tasks.dot
diff --git a/pkg/analyzer/tool/task_dependency_graph/tasks.dot b/pkg/analyzer/tool/task_dependency_graph/tasks.dot
index ad9c3a746edd6f06f9b4f7bcf5005f821dcea0c4..bfc3eadf0126bd585705a4364f3a97a60e4a3bfd 100644
--- a/pkg/analyzer/tool/task_dependency_graph/tasks.dot
+++ b/pkg/analyzer/tool/task_dependency_graph/tasks.dot
@@ -194,6 +194,7 @@ digraph G {
   ParseDartTask -> LIBRARY_SPECIFIC_UNITS
   ParseDartTask -> PARSED_UNIT
   ParseDartTask -> PARSE_ERRORS
+  ParseDartTask -> REFERENCED_SOURCES
   ParseDartTask -> SOURCE_KIND
   ParseDartTask -> UNITS
   PartiallyResolveUnitReferencesTask -> CREATED_RESOLVED_UNIT6
@@ -218,6 +219,8 @@ digraph G {
   READY_RESOLVED_UNIT -> VerifyUnitTask
   READY_RESOLVED_UNIT [shape=box]
   REFERENCED_NAMES [shape=box]
+  REFERENCED_SOURCES -> BuildDirectiveElementsTask
+  REFERENCED_SOURCES [shape=box]
   RESOLVED_UNIT -> GenerateHintsTask
   RESOLVED_UNIT -> GenerateLintsTask
   RESOLVED_UNIT -> ReadyResolvedUnitTask