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

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 945408496277080ee1dcc57f511395d3b1e84544..75e898956876871fd040f6ec91d0e247e58e595f 100644
--- a/pkg/analyzer/tool/task_dependency_graph/tasks.dot
+++ b/pkg/analyzer/tool/task_dependency_graph/tasks.dot
@@ -175,6 +175,7 @@ digraph G {
   MODIFICATION_TIME -> BuildDirectiveElementsTask
   MODIFICATION_TIME -> ParseDartTask
   MODIFICATION_TIME -> ScanDartTask
+  MODIFICATION_TIME -> VerifyUnitTask
   MODIFICATION_TIME [shape=box]
   PARSED_UNIT -> BuildCompilationUnitElementTask
   PARSED_UNIT -> DartErrorsTask
@@ -221,6 +222,7 @@ digraph G {
   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