Make copy of the drivers list in SearchEngineImpl2 to avoid ConcurrentModificationError.

pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart

Index: pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
diff --git a/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart b/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
index d9088a9d9d3c2aa98ca52cdd8facd8c42f0c6ebe..6f1336fd2b5c93a5422935875c8dc0bb2f20fd1e 100644
--- a/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
+++ b/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
@@ -39,7 +39,8 @@ class SearchEngineImpl2 implements SearchEngine {
   @override
   Future<List<SearchMatch>> searchMemberDeclarations(String name) async {
     List<SearchMatch> allDeclarations = [];
-    for (AnalysisDriver driver in _drivers) {
+    List<AnalysisDriver> drivers = _drivers.toList();
+    for (AnalysisDriver driver in drivers) {
       List<Element> elements = await driver.search.classMembers(name);
       allDeclarations.addAll(elements.map(_SearchMatch.forElement));
     }
@@ -49,7 +50,8 @@ class SearchEngineImpl2 implements SearchEngine {
   @override
   Future<List<SearchMatch>> searchMemberReferences(String name) async {
     List<SearchResult> allResults = [];
-    for (AnalysisDriver driver in _drivers) {
+    List<AnalysisDriver> drivers = _drivers.toList();
+    for (AnalysisDriver driver in drivers) {
       List<SearchResult> results =
           await driver.search.unresolvedMemberReferences(name);
       allResults.addAll(results);
@@ -60,7 +62,8 @@ class SearchEngineImpl2 implements SearchEngine {
   @override
   Future<List<SearchMatch>> searchReferences(Element element) async {
     List<SearchResult> allResults = [];
-    for (AnalysisDriver driver in _drivers) {
+    List<AnalysisDriver> drivers = _drivers.toList();
+    for (AnalysisDriver driver in drivers) {
       List<SearchResult> results = await driver.search.references(element);
       allResults.addAll(results);
     }
@@ -77,7 +80,8 @@ class SearchEngineImpl2 implements SearchEngine {
   Future<List<SearchMatch>> searchTopLevelDeclarations(String pattern) async {
     List<SearchMatch> allDeclarations = [];
     RegExp regExp = new RegExp(pattern);
-    for (AnalysisDriver driver in _drivers) {
+    List<AnalysisDriver> drivers = _drivers.toList();
+    for (AnalysisDriver driver in drivers) {
       List<Element> elements = await driver.search.topLevelElements(regExp);
       allDeclarations.addAll(elements.map(_SearchMatch.forElement));
     }
@@ -86,7 +90,8 @@ class SearchEngineImpl2 implements SearchEngine {
 
   Future<List<SearchResult>> _searchDirectSubtypes(ClassElement type) async {
     List<SearchResult> allResults = [];
-    for (AnalysisDriver driver in _drivers) {
+    List<AnalysisDriver> drivers = _drivers.toList();
+    for (AnalysisDriver driver in drivers) {
       List<SearchResult> results = await driver.search.subTypes(type);
       allResults.addAll(results);
     }