Treat type inference context `dynamic` as equivalent to an empty context.

pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
index 32d872cc7e3401920322f94c57ed3ad3a40d323f..16beabb7bf8e4b37d3da6afa9aebed4077fb3836 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
@@ -226,6 +226,13 @@ class TypeSchemaEnvironment extends TypeEnvironment {
       List<DartType> actualTypes,
       DartType returnContextType,
       List<DartType> inferredTypes) {
+    if (returnContextType is DynamicType) {
+      // Analyzer treats a type context of `dynamic` as equivalent to an empty
+      // context.  TODO(paulberry): this is not spec'ed anywhere; do we still
+      // want to do this?
+      returnContextType = null;
+    }
+
     if (typeParametersToInfer.isEmpty) {
       return;
     }