Use file/offset/kind triplet as elements id.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.element;
 
 import 'dart:collection';
@@ skipping 6748 matching lines @@
    * the `part` directive.
    */
   List<CompilationUnitElement> get units;
 
   /**
    * Return a list containing all directly and indirectly imported libraries.
    */
   List<LibraryElement> get visibleLibraries;
 
   /**
+   * Return the element at the given [offset], maybe `null` if no such element.
+   */
+  Element getElementAt(int offset);
+
+  /**
    * Return a list containing all of the imports that share the given [prefix],
    * or an empty array if there are no such imports.
    */
   List<ImportElement> getImportsWithPrefix(PrefixElement prefix);
 
   /**
    * Return the class defined in this library that has the given [name], or
    * `null` if this library does not define a class with the given name.
    */
   ClassElement getType(String className);
@@ skipping 49 matching lines @@
    */
   List<CompilationUnitElement> _parts = CompilationUnitElementImpl.EMPTY_ARRAY;
 
   /**
    * The element representing the synthetic function `loadLibrary` that is
    * defined for this library, or `null` if the element has not yet been created.
    */
   FunctionElement _loadLibraryFunction;
 
   /**
+   * A map from offsets to elements of this library at these offsets.
+   */
+  final Map<int, Element> _elementMap = new HashMap<int, Element>();
+
+  /**
    * The export [Namespace] of this library, `null` if it has not been
    * computed yet.
    */
   @override
   Namespace exportNamespace;
 
   /**
    * The public [Namespace] of this library, `null` if it has not been
    * computed yet.
    */
@@ skipping 279 matching lines @@
     }
     for (ExportElement exportElement in _exports) {
       if ((exportElement as ExportElementImpl).identifier == identifier) {
         return exportElement as ExportElementImpl;
       }
     }
     return null;
   }
 
   @override
+  Element getElementAt(int offset) {
+    if (_elementMap.isEmpty) {
+      accept(new _BuildOffsetToElementMap(_elementMap));
+    }
+    return _elementMap[offset];
+  }
+
+  @override
   List<ImportElement> getImportsWithPrefix(PrefixElement prefixElement) {
     int count = _imports.length;
     List<ImportElement> importList = new List<ImportElement>();
     for (int i = 0; i < count; i++) {
       if (identical(_imports[i].prefix, prefixElement)) {
         importList.add(_imports[i]);
       }
     }
     return importList;
   }
@@ skipping 2951 matching lines @@
     // void <: void (by reflexivity)
     // bottom <: void (as bottom is a subtype of all types).
     // void <: dynamic (as dynamic is a supertype of all types)
     return identical(type, this) || type.isDynamic;
   }
 
   @override
   VoidTypeImpl substitute2(
       List<DartType> argumentTypes, List<DartType> parameterTypes) => this;
 }
+
+/**
+ * A visitor that visit all the elements recursively and fill the given [map].
+ */
+class _BuildOffsetToElementMap extends GeneralizingElementVisitor {
+  final Map<int, Element> map;
+
+  _BuildOffsetToElementMap(this.map);
+
+  @override
+  void visitElement(Element element) {
+    int offset = element.nameOffset;
+    if (offset != -1) {
+      map[offset] = element;
+    }
+    super.visitElement(element);
+  }
+}