Make expansion panel have a minHeight instead (#18623)

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

@@ -143,8 +143,8 @@ class ExpansionPanelList extends StatelessWidget {
               duration: animationDuration,
               curve: Curves.fastOutSlowIn,
               margin: _isChildExpanded(index) ? kExpandedEdgeInsets : EdgeInsets.zero,
-              child: new SizedBox(
-                height: _kPanelHeaderCollapsedHeight,
+              child: new ConstrainedBox(
+                constraints: const BoxConstraints(minHeight: _kPanelHeaderCollapsedHeight),
                 child: children[index].headerBuilder(
                   context,
                   children[index].isExpanded,

packages/flutter/test/material/expansion_panel_test.dart

@@ -126,17 +126,25 @@ void main() {
               animationDuration: kSizeAnimationDuration,
               children: <ExpansionPanel>[
                 new ExpansionPanel(
-                  headerBuilder: (BuildContext context, bool isExpanded) => const Placeholder(),
-                  body: const SizedBox(height: 100.0, child: const Placeholder()),
+                  headerBuilder: (BuildContext context, bool isExpanded) => const Placeholder(
+                    fallbackHeight: 12.0,
+                  ),
+                  body: const SizedBox(height: 100.0, child: const Placeholder(
+                    fallbackHeight: 12.0,
+                  )),
                   isExpanded: a,
                 ),
                 new ExpansionPanel(
-                  headerBuilder: (BuildContext context, bool isExpanded) => const Placeholder(),
+                  headerBuilder: (BuildContext context, bool isExpanded) => const Placeholder(
+                    fallbackHeight: 12.0,
+                  ),
                   body: const SizedBox(height: 100.0, child: const Placeholder()),
                   isExpanded: b,
                 ),
                 new ExpansionPanel(
-                  headerBuilder: (BuildContext context, bool isExpanded) => const Placeholder(),
+                  headerBuilder: (BuildContext context, bool isExpanded) => const Placeholder(
+                    fallbackHeight: 12.0,
+                  ),
                   body: const SizedBox(height: 100.0, child: const Placeholder()),
                   isExpanded: c,
                 ),