Fixes for import/export directive resolution and tasks graph.

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 2834 matching lines @@
    * has the given [source]. Throw an [ElementMismatchException] if an element
    * corresponding to the identifier cannot be found.
    */
   ExportElement _findExport(
       ExportDirective node, List<ExportElement> exports, Source source) {
     if (source == null || !_enclosingUnit.context.exists(source)) {
       return null;
     }
     for (ExportElement export in exports) {
       if (export.exportedLibrary.source == source) {
+        // Must have the same offset.
+        if (export.nameOffset != node.offset) {
+          continue;
+        }
+        // In general we should also match combinators.
+        // But currently we invalidate element model on any directive change.
+        // So, either the combinators are the same, or we build new elements.
         return export;
       }
     }
     _mismatch("Could not find export element for '$source'", node);
     return null; // Never reached
   }
 
   /**
    * Return the import element from the given list of [imports] whose library
    * has the given [source]. Throw an [ElementMismatchException] if an element
    * corresponding to the [source] cannot be found.
    */
   ImportElement _findImport(
       ImportDirective node, List<ImportElement> imports, Source source) {
     if (source == null || !_enclosingUnit.context.exists(source)) {
       return null;
     }
     SimpleIdentifier prefix = node.prefix;
     bool foundSource = false;
     for (ImportElement element in imports) {
       if (element.importedLibrary.source == source) {
         foundSource = true;
-        PrefixElement prefixElement = element.prefix;
-        if (prefix == null) {
-          if (prefixElement == null) {
-            return element;
-          }
-        } else {
-          if (prefixElement != null &&
-              prefix.name == prefixElement.displayName) {
-            return element;
-          }
+        // Must have the same offset.
+        if (element.nameOffset != node.offset) {
+          continue;
         }
+        // Must have the same prefix.
+        if (element.prefix?.displayName != prefix?.name) {
+          continue;
+        }
+        // In general we should also match combinators.
+        // But currently we invalidate element model on any directive change.
+        // So, either the combinators are the same, or we build new elements.
+        return element;
       }
     }
     if (foundSource) {
       if (prefix == null) {
         _mismatch(
             "Could not find import element for '$source' with no prefix", node);
       }
       _mismatch(
           "Could not find import element for '$source' with prefix ${prefix.name}",
           node);
@@ skipping 10071 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }