Cache PrefixElement(s) in LibraryElementImpl.

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 721918f655b3c41c20556433c6c7fbce2d5c2916..2a47fda0350bcd44a61f0190d82b9907ddbe38ba 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -3750,7 +3750,7 @@ class ImportElementImpl extends UriReferencedElementImpl
 
   PrefixElement get prefix {
     if (_unlinkedImport != null) {
-      if (_unlinkedImport.prefixReference != 0) {
+      if (_unlinkedImport.prefixReference != 0 && _prefix == null) {
         LibraryElementImpl library = enclosingElement as LibraryElementImpl;
         _prefix = new PrefixElementImpl.forSerialized(_unlinkedImport, library);
       }
@@ -3976,6 +3976,11 @@ class LibraryElementImpl extends ElementImpl implements LibraryElement {
   Namespace _publicNamespace;
 
   /**
+   * The cached list of prefixes.
+   */
+  List<PrefixElement> _prefixes;
+
+  /**
    * Initialize a newly created library element in the given [context] to have
    * the given [name] and [offset].
    */
@@ -4331,14 +4336,17 @@ class LibraryElementImpl extends ElementImpl implements LibraryElement {
 
   @override
   List<PrefixElement> get prefixes {
-    HashSet<PrefixElement> prefixes = new HashSet<PrefixElement>();
-    for (ImportElement element in _imports) {
-      PrefixElement prefix = element.prefix;
-      if (prefix != null) {
-        prefixes.add(prefix);
+    if (_prefixes == null) {
+      HashSet<PrefixElement> prefixes = new HashSet<PrefixElement>();
+      for (ImportElement element in _imports) {
+        PrefixElement prefix = element.prefix;
+        if (prefix != null) {
+          prefixes.add(prefix);
+        }
       }
+      _prefixes = prefixes.toList();
     }
-    return new List.from(prefixes);
+    return _prefixes;
   }
 
   @override