[proposal] fix assert for proposal builder

Tested: fx ota && check for (lack of) exceptions from krohnkite in the
logs.
Change-Id: Idb8f8f5e880c28dfcb0300a2423a3f08333251c4

Change-Id: Ib40353dd6f2a7d58806ce3d186ed8c33df0f8c44
diff --git a/public/lib/proposal/dart/lib/src/proposal_builder.dart b/public/lib/proposal/dart/lib/src/proposal_builder.dart
index 9aa6d02..c49fade 100644
--- a/public/lib/proposal/dart/lib/src/proposal_builder.dart
+++ b/public/lib/proposal/dart/lib/src/proposal_builder.dart
@@ -25,7 +25,7 @@
     @required String id,
     @required this.headline,
   })  : assert(id != null && id.isNotEmpty),
-        assert(headline != null && headline.isNotEmpty),
+        assert(headline != null),
         _id = id;
 
   /// The commands that will be executed if the proposal is accepted.
@@ -110,6 +110,7 @@
       url: imageUrl,
       imageType: imageType,
     );
+    assert(headline.isNotEmpty || wantsRichSuggestion);
 
     return Proposal(
         id: _id,
diff --git a/public/lib/proposal/dart/lib/src/proposal_builder_async.dart b/public/lib/proposal/dart/lib/src/proposal_builder_async.dart
index c162986..ec46f9e 100644
--- a/public/lib/proposal/dart/lib/src/proposal_builder_async.dart
+++ b/public/lib/proposal/dart/lib/src/proposal_builder_async.dart
@@ -25,7 +25,7 @@
     @required String id,
     @required this.headline,
   })  : assert(id != null && id.isNotEmpty),
-        assert(headline != null && headline.isNotEmpty),
+        assert(headline != null),
         _id = id;
 
   /// The commands that will be executed if the proposal is accepted.
@@ -110,6 +110,7 @@
       url: imageUrl,
       imageType: imageType,
     );
+    assert(headline.isNotEmpty || wantsRichSuggestion);
 
     return Proposal(
         id: _id,