Clear static ElementFactory state at the start of all resolver tests.

pkg/analyzer/lib/src/generated/testing/element_factory.dart

Index: pkg/analyzer/lib/src/generated/testing/element_factory.dart
diff --git a/pkg/analyzer/lib/src/generated/testing/element_factory.dart b/pkg/analyzer/lib/src/generated/testing/element_factory.dart
index d5ac50fb1dd0b1566867430dfe3a68c210565f01..2beeab48e4f67018cb3d8a3fccd0d997a665fc5e 100644
--- a/pkg/analyzer/lib/src/generated/testing/element_factory.dart
+++ b/pkg/analyzer/lib/src/generated/testing/element_factory.dart
@@ -251,6 +251,15 @@ class ElementFactory {
           Identifier name) =>
       new FieldFormalParameterElementImpl.forNode(name);
 
+  /**
+   * Destroy any static state retained by [ElementFactory].  This should be
+   * called from the `setUp` method of any tests that use [ElementFactory], in
+   * order to ensure that state is not shared between multiple tests.
+   */
+  static void flushStaticState() {
+    _objectElement = null;
+  }
+
   static FunctionElementImpl functionElement(String functionName) =>
       functionElement4(functionName, null, null, null, null);