Add asserts to DefaultTextStyle.inherit (#3844)

The context and child arguments are required.  We should assert that
they're non-null.

Fixes #3843

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

diff --git a/packages/flutter/lib/src/widgets/basic.dart b/packages/flutter/lib/src/widgets/basic.dart
index 26b1798..fc7e18d 100644
--- a/packages/flutter/lib/src/widgets/basic.dart
+++ b/packages/flutter/lib/src/widgets/basic.dart
@@ -1994,6 +1994,8 @@
     TextOverflow overflow,
     Widget child
   }) {
+    assert(context != null);
+    assert(child != null);
     DefaultTextStyle parent = DefaultTextStyle.of(context);
     return new DefaultTextStyle(
       key: key,