Don't use AnalysisContext.exists() in VerifyUnitTask.

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 157 matching lines @@
   LINE_INFO -> DartErrorsTask
   LINE_INFO -> ParseDartTask
   LINE_INFO [shape=box]
   LINTS -> LibraryUnitErrorsTask
   LINTS [shape=box]
   LibraryErrorsReadyTask -> LIBRARY_ERRORS_READY
   LibraryUnitErrorsTask -> LIBRARY_UNIT_ERRORS
   MODIFICATION_TIME -> BuildDirectiveElementsTask
   MODIFICATION_TIME -> ParseDartTask
   MODIFICATION_TIME -> ScanDartTask
+  MODIFICATION_TIME -> VerifyUnitTask
   MODIFICATION_TIME [shape=box]
   PARSED_UNIT -> BuildCompilationUnitElementTask
   PARSED_UNIT -> DartErrorsTask
   PARSED_UNIT [shape=box]
   PARSE_ERRORS -> dartErrorsForSource
   PARSE_ERRORS [shape=box]
   PROPAGABLE_VARIABLES_IN_UNIT -> PropagateVariableTypesInUnitTask
   PROPAGABLE_VARIABLES_IN_UNIT [shape=box]
   PROPAGABLE_VARIABLE_DEPENDENCIES -> PropagateVariableTypeTask
   PROPAGABLE_VARIABLE_DEPENDENCIES [shape=box]
@@ skipping 26 matching lines @@
   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 -> VerifyUnitTask
   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]
@@ skipping 94 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]
 }