UIP-1637 First pass at strong mode compliance

.analysis_options

@@ -1,6 +1,5 @@
 analyzer:
-  exclude:
-    - integrate/**
+  strong-mode: true
 linter:
   rules:
     - annotate_overrides

integrate/lib/test_component_declaration.dart

@@ -14,8 +14,7 @@
 
 library test_component_declaration;
 
-import 'package:over_react/ui_core.dart';
-import 'package:over_react/ui_components.dart';
+import 'package:over_react/over_react.dart';
 
 @Factory()
 UiFactory<FooProps> Foo;
@@ -30,7 +29,7 @@ class FooState extends UiState {}
 class FooComponent extends UiStatefulComponent<FooProps, FooState> {
   @override
   render() {
-    return Button()(
+    return Dom.div()(
       'Child 1',
       'Child 2'
     );

integrate/lib/test_react_element_typing.dart

@@ -20,7 +20,14 @@ import './test_component_declaration.dart';
 
 /// Verify that treating the invocation of the builder as an unparameterized [ReactElement]
 /// does not result in the following analyzer warning:
-/// > Unsound implicit cast from ReactElement<dynamic> to ReactElement<Component>
-void _reactElementTypingTest() {
+///
+///     Unsound implicit cast from ReactElement<dynamic> to ReactElement<Component>
+ReactElement _reactElementTypingTest() {
   ReactElement element = Foo()();
+
+  return element;
+}
+
+main() {
+  _reactElementTypingTest();
 }