Enable avoid_as lint (#3612)

Related to #3608

packages/flutter_test/lib/src/widget_tester.dart

@@ -118,8 +118,10 @@ class WidgetTester {
     Element element = finder.findFirst(this);
     Widget widget = element.widget;
 
-    if (widget is StatefulWidget)
-      return (element as StatefulElement).state;
+    if (widget is StatefulWidget) {
+      StatefulElement statefulElement = element;
+      return statefulElement.state;
+    }
 
     throw new ElementNotFoundError(
       'Widget of type ${widget.runtimeType} found by ${finder.description} does not correspond to a StatefulWidget'

packages/flutter_tools/flutter_analysis_options

@@ -29,7 +29,7 @@ linter:
     - always_declare_return_types
     - always_specify_types
     - annotate_overrides
-    # - avoid_as # https://github.com/dart-lang/linter/issues/195
+    - avoid_as
     - avoid_init_to_null
     # - avoid_return_types_on_setters # https://github.com/dart-lang/linter/issues/202
     - camel_case_types

packages/flutter_tools/lib/src/cache.dart

@@ -143,7 +143,8 @@ class Cache {
       os.unzip(tempFile, location);
       tempFile.deleteSync();
     } else {
-      (location as File).writeAsBytesSync(fileBytes, flush: true);
+      File file = location;
+      file.writeAsBytesSync(fileBytes, flush: true);
     }
   }
 }