Restore implementation class for DOMStringMap.

lib/dom/src/native_DOMImplementation.dart

Index: lib/dom/src/native_DOMImplementation.dart
diff --git a/lib/dom/src/native_DOMImplementation.dart b/lib/dom/src/native_DOMImplementation.dart
index c9e98d0278e72b2831db4747690c411ecdfeeac6..407d3e1c27582c45d49ab6c985bb5235449aa086 100644
--- a/lib/dom/src/native_DOMImplementation.dart
+++ b/lib/dom/src/native_DOMImplementation.dart
@@ -87,3 +87,20 @@ class _LocationCrossFrameImpl extends _DOMWrapperBase implements Location {
   // Implementation support.
   String get typeName() => "Location";
 }
+
+class _DOMStringMapImpl extends _DOMWrapperBase implements Map<String, String> {
+  _DOMStringMapImpl();
+
+  bool containsValue(String value) => Maps.containsValue(this, value);
+  bool containsKey(String key) native "DOMStringMap_containsKey_Callback";
+  String operator [](String key) native "DOMStringMap_item_Callback";
+  void operator []=(String key, String value) native "DOMStringMap_setItem_Callback";
+  String putIfAbsent(String key, String ifAbsent()) => Maps.putIfAbsent(this, key, ifAbsent);
+  String remove(String key) native "DOMStringMap_remove_Callback";
+  void clear() => Maps.clear(this);
+  void forEach(void f(String key, String value)) => Maps.forEach(this, f);
+  Collection<String> getKeys() native "DOMStringMap_getKeys_Callback";
+  Collection<String> getValues() => Maps.getValues(this);
+  int get length() => Maps.length(this);
+  bool isEmpty() => Maps.isEmpty(this);
+}