Fixes a problem with the height of the chip label (#16420)

There was a bug in the size calculation for the height of the chip label, where it would force the label to be larger than it wanted to be, causing text to not be vertically centered.

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

@@ -1892,22 +1892,33 @@ class _RenderChip extends RenderBox {
   }
 
   Size _layoutLabel(double iconSizes, Size size) {
+    final Size rawSize = _boxSize(label);
     // Now that we know the label height and the width of the icons, we can
     // determine how much to shrink the width constraints for the "real" layout.
     if (constraints.maxWidth.isFinite) {
       label.layout(
-        constraints.loosen().copyWith(
+        constraints.copyWith(
+          minWidth: 0.0,
           maxWidth: math.max(
             0.0,
             constraints.maxWidth - iconSizes - theme.labelPadding.horizontal,
           ),
-            maxHeight: size.height),
+          minHeight: rawSize.height,
+          maxHeight: size.height,
+        ),
         parentUsesSize: true,
       );
     } else {
-      label.layout(new BoxConstraints.tight(size), parentUsesSize: true);
+      label.layout(
+        new BoxConstraints(
+          minHeight: rawSize.height,
+          maxHeight: size.height,
+          minWidth: 0.0,
+          maxWidth: size.width,
+        ),
+        parentUsesSize: true,
+      );
     }
-    final Size rawSize = _boxSize(label);
     return new Size(
       rawSize.width + theme.labelPadding.horizontal,
       rawSize.height + theme.labelPadding.vertical,
@@ -2064,7 +2075,6 @@ class _RenderChip extends RenderBox {
         } else {
           deleteButtonRect = Rect.zero;
         }
-        //assert(start + deleteIconSize.width == overallSize.width);
         break;
     }
     // Center the label vertically.

packages/flutter/test/material/chip_test.dart

@@ -200,7 +200,7 @@ void main() {
         ),
       ),
     );
-    expect(tester.getSize(find.byType(Text)), const Size(40.0, 24.0));
+    expect(tester.getSize(find.byType(Text)), const Size(40.0, 10.0));
     expect(tester.getSize(find.byType(Chip)), const Size(64.0, 32.0));
     await tester.pumpWidget(
       new MaterialApp(