Remove many explicit references to AnalysisContextImpl and enable testing of the new task model

pkg/analyzer/test/task/task_dart_test.dart

Index: pkg/analyzer/test/task/task_dart_test.dart
diff --git a/pkg/analyzer/test/task/task_dart_test.dart b/pkg/analyzer/test/task/task_dart_test.dart
index 352bc3bc30450aaffd9b239d31c06bc29e077eba..e7c03bec5bdec9565964f6669d4b27baec19b996 100644
--- a/pkg/analyzer/test/task/task_dart_test.dart
+++ b/pkg/analyzer/test/task/task_dart_test.dart
@@ -63,8 +63,7 @@ class BuildUnitElementTaskTest extends EngineTestCase {
   void test_perform_exception() {
     TestSource source = new TestSource();
     source.generateExceptionOnRead = true;
-    InternalAnalysisContext context = new AnalysisContextImpl();
-    context.sourceFactory = new SourceFactory([new FileUriResolver()]);
+    InternalAnalysisContext context = AnalysisContextFactory.contextWithCore();
     CompilationUnit unit = parseUnit(context, source, "");
     BuildUnitElementTask task =
         new BuildUnitElementTask(context, null, source, unit);