Compute suggestions for failed type promotions.

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index eeffacb071831cf14b7495b09fa6f10f2ca6e17a..c957de3426b04e66d0478966c0d10e0feaf57a30 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -1394,6 +1394,17 @@ Please include the following information:
       "'#{shownType}' because '#{shownType}' is not more specific than the "
       "known type '#{knownType}' of '#{variableName}'.");
 
+  static const MessageKind NOT_MORE_SPECIFIC_SUBTYPE = const MessageKind(
+      "Hint: Variable '#{variableName}' is not shown to have type "
+      "'#{shownType}' because '#{shownType}' is not a subtype of the "
+      "known type '#{knownType}' of '#{variableName}'.");
+
+  static const MessageKind NOT_MORE_SPECIFIC_SUGGESTION = const MessageKind(
+      "Hint: Variable '#{variableName}' is not shown to have type "
+      "'#{shownType}' because '#{shownType}' is not more specific than the "
+      "known type '#{knownType}' of '#{variableName}'.",
+      howToFix: "Try replacing '#{shownType}' with '#{shownTypeSuggestion}'.");
+
   //////////////////////////////////////////////////////////////////////////////
   // Patch errors start.
   //////////////////////////////////////////////////////////////////////////////