Use single map of Source to modification time.

pkg/analyzer/tool/task_dependency_graph/tasks.dot

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // This file has been automatically generated.  Please do not edit it manually.
 // To regenerate the file, use the script
 // "pkg/analyzer/tool/task_dependency_graph/generate.dart".
 //
 // To render this graph using Graphviz (www.graphviz.org) use the command:
 // "dot tasks.dot -Tpdf -O".
@@ skipping 176 matching lines @@
   PROPAGATED_VARIABLE -> PropagateVariableTypeTask
   PROPAGATED_VARIABLE -> PropagateVariableTypesInUnitTask
   PROPAGATED_VARIABLE [shape=box]
   ParseDartTask -> EXPLICITLY_IMPORTED_LIBRARIES
   ParseDartTask -> EXPORTED_LIBRARIES
   ParseDartTask -> IMPORTED_LIBRARIES
   ParseDartTask -> INCLUDED_PARTS
   ParseDartTask -> LIBRARY_SPECIFIC_UNITS
   ParseDartTask -> PARSED_UNIT
   ParseDartTask -> PARSE_ERRORS
+  ParseDartTask -> REFERENCED_SOURCES
   ParseDartTask -> SOURCE_KIND
   ParseDartTask -> UNITS
   PartiallyResolveUnitReferencesTask -> CREATED_RESOLVED_UNIT6
   PartiallyResolveUnitReferencesTask -> INFERABLE_STATIC_VARIABLES_IN_UNIT
   PartiallyResolveUnitReferencesTask -> PROPAGABLE_VARIABLES_IN_UNIT
   PartiallyResolveUnitReferencesTask -> RESOLVED_UNIT6
   PropagateVariableTypeTask -> PROPAGATED_VARIABLE
   PropagateVariableTypesInLibraryClosureTask -> LIBRARY_ELEMENT7
   PropagateVariableTypesInLibraryTask -> LIBRARY_ELEMENT6
   PropagateVariableTypesInUnitTask -> CREATED_RESOLVED_UNIT7
   PropagateVariableTypesInUnitTask -> RESOLVED_UNIT7
   READY_LIBRARY_ELEMENT2 -> ComputeLibraryCycleTask
   READY_LIBRARY_ELEMENT2 -> ReadyLibraryElement2Task
   READY_LIBRARY_ELEMENT2 [shape=box]
   READY_LIBRARY_ELEMENT5 -> PartiallyResolveUnitReferencesTask
   READY_LIBRARY_ELEMENT5 -> ReadyLibraryElement5Task
   READY_LIBRARY_ELEMENT5 [shape=box]
   READY_LIBRARY_ELEMENT6 -> PropagateVariableTypesInLibraryClosureTask
   READY_LIBRARY_ELEMENT6 -> ReadyLibraryElement6Task
   READY_LIBRARY_ELEMENT6 [shape=box]
   READY_RESOLVED_UNIT -> ResolveLibraryTask
   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
   RESOLVED_UNIT -> VerifyUnitTask
   RESOLVED_UNIT [shape=box]
   RESOLVED_UNIT1 -> BuildDirectiveElementsTask
   RESOLVED_UNIT1 -> BuildLibraryElementTask
   RESOLVED_UNIT1 -> ResolveDirectiveElementsTask
   RESOLVED_UNIT1 [shape=box]
   RESOLVED_UNIT10 -> ResolveUnitTask
@@ skipping 93 matching lines @@
   VARIABLE_REFERENCE_ERRORS -> LibraryUnitErrorsTask
   VARIABLE_REFERENCE_ERRORS [shape=box]
   VERIFY_ERRORS -> LibraryUnitErrorsTask
   VERIFY_ERRORS [shape=box]
   VerifyUnitTask -> VERIFY_ERRORS
   dartErrorsForSource -> DartErrorsTask
   dartErrorsForSource [shape=hexagon]
   dartErrorsForUnit -> DartErrorsTask
   dartErrorsForUnit [shape=hexagon]
 }