Add a hint on static uses of fuzzy arrows.

pkg/analyzer/lib/src/task/strong/checker.dart

Index: pkg/analyzer/lib/src/task/strong/checker.dart
diff --git a/pkg/analyzer/lib/src/task/strong/checker.dart b/pkg/analyzer/lib/src/task/strong/checker.dart
index e09f6af3f3b8fec50d6c82c46e312c3ac9826b7d..e6e2b34f0dc0f66dcf6f1704bd0886e9806be61b 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -772,6 +772,8 @@ class CodeChecker extends RecursiveAstVisitor {
       {DartType from, bool opAssign: false, bool isDeclarationCast: false}) {
     from ??= _getDefiniteType(expr);
 
+    _hintOnFuzzyArrows(expr, to, from: from);
+
     if (_needsImplicitCast(expr, to,
             from: from, isDeclarationCast: isDeclarationCast) ==
         true) {
@@ -1075,6 +1077,23 @@ class CodeChecker extends RecursiveAstVisitor {
     return rules.anyParameterType(ft, (pt) => pt.isDynamic);
   }
 
+  void _hintOnFuzzyArrows(Expression expr, DartType to, {DartType from}) {
+    from ??= _getDefiniteType(expr);

[Jennifer Messerly, 2017/08/30 23:29:48]

this is not needed because `_checkImplicitCast` did this before this function
was called

Also the `expr` parameter is not necessary.

+
+    // If it is a subtype with fuzzy arrows on,
+    // check to see if it still is with them off.
+    if (rules.isSubtypeOf(from, to)) {

[Jennifer Messerly, 2017/08/30 23:29:48]

alternate way to implement this method that may be simpler:

// If it is a subtype with fuzzy arrows on,
/ check to see if it still is with them off.
if (rules.isSubtypeOf(from, to)) {
  // Remove fuzzy arrows
  var cFrom = rules.typeToConcreteType(from);
  var cTo = rules.typeToConcreteType(to);
  // If still true, no warning needed
  if (rules.isSubtypeOf(cFrom, cTo)) return;
  _recordMessage(expr, HintCode.USES_DYNAMIC_AS_BOTTOM, [from, to]);
}

+      try {
+        rules.allowDynamicAsBottom = false;
+        // If still true, no warning needed
+        if (rules.isSubtypeOf(from, to)) return;
+        _recordMessage(expr, HintCode.USES_DYNAMIC_AS_BOTTOM, [from, to]);
+      } finally {
+        rules.allowDynamicAsBottom = true;
+      }
+    }
+  }
+
   /// Returns true if we need an implicit cast of [expr] from [from] type to
   /// [to] type, returns false if no cast is needed, and returns null if the
   /// types are statically incompatible.
@@ -1224,7 +1243,9 @@ class CodeChecker extends RecursiveAstVisitor {
         errorCode.name.startsWith('STRONG_MODE_TOP_LEVEL_')) {
       severity = ErrorSeverity.ERROR;
     }
-    if (severity != ErrorSeverity.INFO || _options.strongModeHints) {
+    if (severity != ErrorSeverity.INFO ||
+        _options.strongModeHints ||
+        errorCode == HintCode.USES_DYNAMIC_AS_BOTTOM) {
       int begin = node is AnnotatedNode
           ? node.firstTokenAfterCommentAndMetadata.offset
           : node.offset;