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

This prevents the ElementFactory's ClassElement for `Object` (which it
stores statically) from getting accidentally shared between separate
unit tests in a single test run, which could cause state to leak
between the tests.

R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org/1752023002 .

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);
 

pkg/analyzer/test/generated/resolver_test.dart

@@ -8497,6 +8497,7 @@ class ResolverTestCase extends EngineTestCase {
 
   @override
   void setUp() {
+    ElementFactory.flushStaticState();
     super.setUp();
     reset();
   }