Fix cache corruption in incremental resolver

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 81f557c958b60ae39e2494037254bf5997056e70..f4f7692188960f16443cd7e19304872dbeabb732 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -1792,8 +1792,7 @@ abstract class ElementImpl implements Element {
    */
   void set enclosingElement(Element element) {
     _enclosingElement = element as ElementImpl;
-    _cachedLocation = null;
-    _cachedHashCode = null;
+    _updateCaches();
   }
 
   @override
@@ -1867,8 +1866,7 @@ abstract class ElementImpl implements Element {
 
   void set name(String name) {
     this._name = name;
-    _cachedLocation = null;
-    _cachedHashCode = null;
+    _updateCaches();
   }
 
   @override
@@ -1883,8 +1881,7 @@ abstract class ElementImpl implements Element {
    */
   void set nameOffset(int offset) {
     _nameOffset = offset;
-    _cachedHashCode = null;
-    _cachedLocation = null;
+    _updateCaches();
   }
 
   @override
@@ -2045,6 +2042,33 @@ abstract class ElementImpl implements Element {
   void visitChildren(ElementVisitor visitor) {
     // There are no children to visit
   }
+
+  /**
+   *  Updates cached values after an input changed.
+   */
+  void _updateCaches() {
+    if (!hasModifier(Modifier.CACHE_KEY)) {
+      // Fast path.
+      _cachedLocation = null;
+      _cachedHashCode = null;
+      return;
+    }
+
+    // Save originals.
+    ElementLocation oldLocation = _cachedLocation;
+    int oldHashCode = _cachedHashCode;
+
+    _cachedLocation = null;
+    _cachedHashCode = null;
+
+    if (oldHashCode != hashCode) {
+      // Prevent cache corruption by restoring originals.
+      _cachedLocation = oldLocation;
+      _cachedHashCode = oldHashCode;
+      throw new StateError(
+          "can't update hashCode for a cache key: $this ($runtimeType)");
+    }
+  }
 }
 
 /**
@@ -3815,7 +3839,12 @@ class Modifier extends Enum<Modifier> {
    */
   static const Modifier SYNTHETIC = const Modifier('SYNTHETIC', 18);
 
-  static const List<Modifier> values = const [
+  /**
+   * Indicates that this element is being used as an analyzer cache key.
+   */
+  static const Modifier CACHE_KEY = const Modifier('CACHE_KEY', 19);
+
+  static const List<Modifier> persistedValues = const [
     ABSTRACT,
     ASYNCHRONOUS,
     CONST,
@@ -3837,6 +3866,11 @@ class Modifier extends Enum<Modifier> {
     SYNTHETIC
   ];
 
+  static const List<Modifier> transientValues = const [CACHE_KEY];
+
+  static final values = new List.unmodifiable(
+      []..addAll(persistedValues)..addAll(transientValues));
+
   const Modifier(String name, int ordinal) : super(name, ordinal);
 }