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

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index fa360f1ac43c3518a04f74793d812bb5b0f22162..3124cd9e9252a035e074d10356158048a659cbd4 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -5313,6 +5313,13 @@ class VerifyUnitTask extends SourceBasedAnalysisTask {
   static const String UNIT_INPUT = 'UNIT_INPUT';
 
   /**
+   * The name of the input of a mapping from [REFERENCED_SOURCES] to their
+   * [MODIFICATION_TIME]s.
+   */
+  static const String REFERENCED_SOURCE_MODIFICATION_TIME_MAP_INPUT =
+      'REFERENCED_SOURCE_MODIFICATION_TIME_MAP_INPUT';
+
+  /**
    * The name of the [TYPE_PROVIDER] input.
    */
   static const String TYPE_PROVIDER_INPUT = 'TYPE_PROVIDER_INPUT';
@@ -5328,6 +5335,12 @@ class VerifyUnitTask extends SourceBasedAnalysisTask {
    */
   ErrorReporter errorReporter;
 
+  /**
+   * The mapping from the current library referenced sources to their
+   * modification times.
+   */
+  Map<Source, int> sourceTimeMap;
+
   VerifyUnitTask(InternalAnalysisContext context, AnalysisTarget target)
       : super(context, target);
 
@@ -5344,6 +5357,8 @@ class VerifyUnitTask extends SourceBasedAnalysisTask {
     //
     TypeProvider typeProvider = getRequiredInput(TYPE_PROVIDER_INPUT);
     CompilationUnit unit = getRequiredInput(UNIT_INPUT);
+    sourceTimeMap =
+        getRequiredInput(REFERENCED_SOURCE_MODIFICATION_TIME_MAP_INPUT);
     CompilationUnitElement unitElement = unit.element;
     LibraryElement libraryElement = unitElement.library;
     if (libraryElement == null) {
@@ -5398,7 +5413,8 @@ class VerifyUnitTask extends SourceBasedAnalysisTask {
   void validateReferencedSource(UriBasedDirective directive) {
     Source source = directive.source;
     if (source != null) {
-      if (context.exists(source)) {
+      int modificationTime = sourceTimeMap[source] ?? -1;
+      if (modificationTime >= 0) {
         return;
       }
     } else {
@@ -5422,6 +5438,8 @@ class VerifyUnitTask extends SourceBasedAnalysisTask {
     return <String, TaskInput>{
       'thisLibraryClosureIsReady': READY_RESOLVED_UNIT.of(unit.library),
       UNIT_INPUT: RESOLVED_UNIT.of(unit),
+      REFERENCED_SOURCE_MODIFICATION_TIME_MAP_INPUT:
+          REFERENCED_SOURCES.of(unit.library).toMapOf(MODIFICATION_TIME),
       TYPE_PROVIDER_INPUT: TYPE_PROVIDER.of(AnalysisContextTarget.request)
     };
   }