Don't get value in SingleMapIterator until it is requested.

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

Index: pkg/analyzer/lib/src/generated/utilities_collection.dart
diff --git a/pkg/analyzer/lib/src/generated/utilities_collection.dart b/pkg/analyzer/lib/src/generated/utilities_collection.dart
index 47dd4f61d46e47033baa2fed7cefb8604f64fa94..bd03a10797372edce52f976e0b3cba565d995d76 100644
--- a/pkg/analyzer/lib/src/generated/utilities_collection.dart
+++ b/pkg/analyzer/lib/src/generated/utilities_collection.dart
@@ -681,6 +681,9 @@ class SingleMapIterator<K, V> implements MapIterator<K, V> {
     if (_currentKey == null) {
       throw new NoSuchElementException();
     }
+    if (_currentValue == null) {
+      _currentValue = _map[_currentKey];
+    }
     return _currentValue;
   }
 
@@ -697,7 +700,7 @@ class SingleMapIterator<K, V> implements MapIterator<K, V> {
   bool moveNext() {
     if (_keyIterator.moveNext()) {
       _currentKey = _keyIterator.current;
-      _currentValue = _map[_currentKey];
+      _currentValue = null;
       return true;
     } else {
       _currentKey = null;