Clarify the "needs an Overlay" assert.

Fixes https://github.com/flutter/flutter/issues/2436

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

@@ -182,6 +182,7 @@ class _DropDownRoute<T> extends PopupRoute<_DropDownRouteResult<T>> {
 
   ModalPosition getPosition(BuildContext context) {
     RenderBox overlayBox = Overlay.of(context).context.findRenderObject();
+    assert(overlayBox != null); // can't be null; routes get inserted by Navigator which has its own Overlay
     Size overlaySize = overlayBox.size;
     RelativeRect menuRect = new RelativeRect.fromSize(rect, overlaySize);
     return new ModalPosition(

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

@@ -46,6 +46,7 @@ class Tooltip extends StatefulComponent {
     assert(preferBelow != null);
     assert(fadeDuration != null);
     assert(showDuration != null);
+    assert(child != null);
   }
 
   final String message;
@@ -150,7 +151,7 @@ class _TooltipState extends State<Tooltip> {
           preferBelow: config.preferBelow
         );
       });
-      Overlay.of(context).insert(_entry);
+      Overlay.of(context, debugRequiredFor: config).insert(_entry);
     }
     _timer?.cancel();
     if (_controller.status != AnimationStatus.completed) {
@@ -175,7 +176,7 @@ class _TooltipState extends State<Tooltip> {
   }
 
   Widget build(BuildContext context) {
-    assert(Overlay.of(context) != null);
+    assert(Overlay.of(context, debugRequiredFor: config) != null);
     return new GestureDetector(
       behavior: HitTestBehavior.opaque,
       onLongPress: showTooltip,

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

@@ -234,7 +234,7 @@ class _DraggableState<T> extends State<DraggableBase<T>> {
       _activeCount += 1;
     });
     return new _DragAvatar<T>(
-      overlay: Overlay.of(context),
+      overlay: Overlay.of(context, debugRequiredFor: config),
       data: config.data,
       initialPosition: position,
       dragStartPoint: dragStartPoint,
@@ -249,6 +249,7 @@ class _DraggableState<T> extends State<DraggableBase<T>> {
   }
 
   Widget build(BuildContext context) {
+    assert(Overlay.of(context, debugRequiredFor: config) != null);
     final bool canDrag = config.maxSimultaneousDrags == null ||
                          _activeCount < config.maxSimultaneousDrags;
     final bool showChild = _activeCount == 0 || config.childWhenDragging == null;

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

@@ -137,6 +137,9 @@ class Mimic extends StatelessComponent {
 }
 
 /// A widget that can be copied by a [Mimic].
+///
+/// This widget's State, [MimicableState], contains an API for initiating the
+/// mimic operation.
 class Mimicable extends StatefulComponent {
   Mimicable({ Key key, this.child }) : super(key: key);
 
@@ -193,8 +196,7 @@ class MimicableState extends State<Mimicable> {
   /// had when the mimicking process started and (2) the child will be
   /// placed in the enclosing overlay.
   MimicOverlayEntry liftToOverlay() {
-    OverlayState overlay = Overlay.of(context);
-    assert(overlay != null); // You need an overlay to lift into.
+    OverlayState overlay = Overlay.of(context, debugRequiredFor: config);
     MimicOverlayEntry entry = new MimicOverlayEntry._(startMimic());
     overlay.insert(entry._overlayEntry);
     return entry;

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

@@ -71,7 +71,32 @@ class Overlay extends StatefulComponent {
   final List<OverlayEntry> initialEntries;
 
   /// The state from the closest instance of this class that encloses the given context.
-  static OverlayState of(BuildContext context) => context.ancestorStateOfType(const TypeMatcher<OverlayState>());
+  ///
+  /// In checked mode, if the [debugRequiredFor] argument is provided then this
+  /// function will assert that an overlay was found and will throw an exception
+  /// if not. The exception attempts to explain that the calling [Widget] (the
+  /// one given by the [debugRequiredFor] argument) needs an [Overlay] to be
+  /// present to function.
+  static OverlayState of(BuildContext context, { Widget debugRequiredFor }) {
+    OverlayState result = context.ancestorStateOfType(const TypeMatcher<OverlayState>());
+    assert(() {
+      if (debugRequiredFor != null && result == null) {
+        String additional = context.widget != debugRequiredFor
+          ? '\nThe context from which that widget was searching for an overlay was:\n  $context'
+          : '';
+        throw new WidgetError(
+          'No Overlay widget found.\n'
+          '${debugRequiredFor.runtimeType} widgets require an Overlay widget ancestor for correct operation.\n'
+          'The most common way to add an Overlay to an application is to include a MaterialApp or Navigator widget in the runApp() call.\n'
+          'The specific widget that failed to find an overlay was:\n'
+          '  $debugRequiredFor'
+          '$additional'
+        );
+      }
+      return true;
+    });
+    return result;
+  }
 
   OverlayState createState() => new OverlayState();
 }