Don't recurse into (external) compilation units.

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

Index: pkg/analyzer/lib/src/task/incremental_element_builder.dart
diff --git a/pkg/analyzer/lib/src/task/incremental_element_builder.dart b/pkg/analyzer/lib/src/task/incremental_element_builder.dart
index 15f02dc33abd328efbd9fa5c78537da41d87b493..d4c107ef42d912fec24537f65853b6446285dd06 100644
--- a/pkg/analyzer/lib/src/task/incremental_element_builder.dart
+++ b/pkg/analyzer/lib/src/task/incremental_element_builder.dart
@@ -273,13 +273,17 @@ class _UpdateElementOffsetsVisitor extends GeneralizingElementVisitor {
 
   _UpdateElementOffsetsVisitor(this.map);
 
-  visitElement(Element element) {
-    int oldOffset = element.nameOffset;
-    if (oldOffset != -1) {
-      int newOffset = map[oldOffset];
-      assert(newOffset != null);
-      (element as ElementImpl).nameOffset = newOffset;
+  void visitElement(Element element) {
+    if (element is CompilationUnitElement) {
+      return;
     }
+    if (element.isSynthetic) {
+      return;
+    }
+    int oldOffset = element.nameOffset;
+    int newOffset = map[oldOffset];
+    assert(newOffset != null);
+    (element as ElementImpl).nameOffset = newOffset;
     if (element is! LibraryElement) {
       super.visitElement(element);
     }