Pave the Foo.of() cowpath.

I'm planning on adding more .of() functions and so to avoid further
copypasta I'm providing some general utility functions here.

packages/flutter/lib/src/material/icon_theme.dart

@@ -19,7 +19,7 @@ class IconTheme extends InheritedWidget {
   final IconThemeData data;
 
   static IconThemeData of(BuildContext context) {
-    IconTheme result = context.inheritedWidgetOfType(IconTheme);
+    IconTheme result = context.inheritFromWidgetOfType(IconTheme);
     return result?.data;
   }
 

packages/flutter/lib/src/material/material_button.dart

@@ -23,7 +23,7 @@ class ButtonTheme extends InheritedWidget {
   final ButtonColor color;
 
   static ButtonColor of(BuildContext context) {
-    ButtonTheme result = context.inheritedWidgetOfType(ButtonTheme);
+    ButtonTheme result = context.inheritFromWidgetOfType(ButtonTheme);
     return result?.color ?? ButtonColor.normal;
   }
 

packages/flutter/lib/src/material/theme.dart

@@ -23,7 +23,7 @@ class Theme extends InheritedWidget {
   static final ThemeData _kFallbackTheme = new ThemeData.fallback();
 
   static ThemeData of(BuildContext context) {
-    Theme theme = context.inheritedWidgetOfType(Theme);
+    Theme theme = context.inheritFromWidgetOfType(Theme);
     return theme == null ? _kFallbackTheme : theme.data;
   }
 

packages/flutter/lib/src/widgets/basic.dart

@@ -972,7 +972,7 @@ class DefaultTextStyle extends InheritedWidget {
   final TextStyle style;
 
   static TextStyle of(BuildContext context) {
-    DefaultTextStyle result = context.inheritedWidgetOfType(DefaultTextStyle);
+    DefaultTextStyle result = context.inheritFromWidgetOfType(DefaultTextStyle);
     return result?.style;
   }
 
@@ -1163,7 +1163,7 @@ class DefaultAssetBundle extends InheritedWidget {
   final AssetBundle bundle;
 
   static AssetBundle of(BuildContext context) {
-    DefaultAssetBundle result = context.inheritedWidgetOfType(DefaultAssetBundle);
+    DefaultAssetBundle result = context.inheritFromWidgetOfType(DefaultAssetBundle);
     return result?.bundle;
   }
 

packages/flutter/lib/src/widgets/focus.dart

@@ -87,7 +87,7 @@ class Focus extends StatefulComponent {
   static bool at(BuildContext context, Widget widget, { bool autofocus: true }) {
     assert(widget != null);
     assert(widget.key is GlobalKey);
-    _FocusScope focusScope = context.inheritedWidgetOfType(_FocusScope);
+    _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
     if (focusScope != null) {
       if (autofocus)
         focusScope._setFocusedWidgetIfUnset(widget.key);
@@ -100,7 +100,7 @@ class Focus extends StatefulComponent {
 
   static bool _atScope(BuildContext context, Widget widget, { bool autofocus: true }) {
     assert(widget != null);
-    _FocusScope focusScope = context.inheritedWidgetOfType(_FocusScope);
+    _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
     if (focusScope != null) {
       if (autofocus)
         focusScope._setFocusedScopeIfUnset(widget.key);
@@ -118,7 +118,7 @@ class Focus extends StatefulComponent {
   static void moveTo(BuildContext context, Widget widget) {
     assert(widget != null);
     assert(widget.key is GlobalKey);
-    _FocusScope focusScope = context.inheritedWidgetOfType(_FocusScope);
+    _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
     if (focusScope != null)
       focusScope.focusState._setFocusedWidget(widget.key);
   }
@@ -126,7 +126,7 @@ class Focus extends StatefulComponent {
   static void moveScopeTo(BuildContext context, Focus component) {
     assert(component != null);
     assert(component.key != null);
-    _FocusScope focusScope = context.inheritedWidgetOfType(_FocusScope);
+    _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
     if (focusScope != null)
       focusScope.focusState._setFocusedScope(component.key);
   }

packages/flutter/lib/src/widgets/framework.dart

@@ -534,7 +534,9 @@ typedef void ElementVisitor(Element element);
 abstract class BuildContext {
   Widget get widget;
   RenderObject findRenderObject();
-  InheritedWidget inheritedWidgetOfType(Type targetType);
+  InheritedWidget inheritFromWidgetOfType(Type targetType);
+  Widget ancestorWidgetOfType(Type targetType);
+  State ancestorStateOfType(Type targetType);
   void visitAncestorElements(bool visitor(Element element));
   void visitChildElements(void visitor(Element element));
 }
@@ -806,18 +808,33 @@ abstract class Element<T extends Widget> implements BuildContext {
     assert(() { _debugLifecycleState = _ElementLifecycle.defunct; return true; });
   }
 
+  RenderObject findRenderObject() => renderObject;
+
   Set<Type> _dependencies;
-  InheritedWidget inheritedWidgetOfType(Type targetType) {
+  InheritedWidget inheritFromWidgetOfType(Type targetType) {
     if (_dependencies == null)
       _dependencies = new Set<Type>();
     _dependencies.add(targetType);
+    return ancestorWidgetOfType(targetType);
+  }
+
+  Widget ancestorWidgetOfType(Type targetType) {
     Element ancestor = _parent;
     while (ancestor != null && ancestor.widget.runtimeType != targetType)
       ancestor = ancestor._parent;
     return ancestor?.widget;
   }
 
-  RenderObject findRenderObject() => renderObject;
+  State ancestorStateOfType(Type targetType) {
+    Element ancestor = _parent;
+    while (ancestor != null) {
+      if (ancestor is StatefulComponentElement && ancestor.state.runtimeType == targetType)
+        break;
+      ancestor = ancestor._parent;
+    }
+    StatefulComponentElement statefulAncestor = ancestor;
+    return statefulAncestor?.state;
+  }
 
   void visitAncestorElements(bool visitor(Element element)) {
     Element ancestor = _parent;

packages/flutter/lib/src/widgets/media_query.dart

@@ -42,7 +42,7 @@ class MediaQuery extends InheritedWidget {
   final MediaQueryData data;
 
   static MediaQueryData of(BuildContext context) {
-    MediaQuery query = context.inheritedWidgetOfType(MediaQuery);
+    MediaQuery query = context.inheritFromWidgetOfType(MediaQuery);
     return query == null ? null : query.data;
   }
 

packages/flutter/lib/src/widgets/navigator.dart

@@ -54,17 +54,7 @@ class Navigator extends StatefulComponent {
 
   static const String defaultRouteName = '/';
 
-  static NavigatorState of(BuildContext context) {
-    NavigatorState result;
-    context.visitAncestorElements((Element element) {
-      if (element is StatefulComponentElement && element.state is NavigatorState) {
-        result = element.state;
-        return false;
-      }
-      return true;
-    });
-    return result;
-  }
+  static NavigatorState of(BuildContext context) => context.ancestorStateOfType(NavigatorState);
 
   NavigatorState createState() => new NavigatorState();
 }

packages/flutter/lib/src/widgets/page_storage.dart

@@ -86,16 +86,8 @@ class PageStorage extends StatelessComponent {
 
   /// Might return null if there is no PageStorage in this context.
   static PageStorageBucket of(BuildContext context) {
-    PageStorageBucket result;
-    context.visitAncestorElements((Element element) {
-      Widget widget = element.widget;
-      if (widget is PageStorage) {
-        result = widget.bucket;
-        return false;
-      }
-      return true;
-    });
-    return result;
+    PageStorage widget = context.ancestorWidgetOfType(PageStorage);
+    return widget?.bucket;
   }
 
   Widget build(BuildContext context) => child;