Fix checked-mode return for async-functions.

pkg/compiler/lib/src/dart_types.dart

Index: pkg/compiler/lib/src/dart_types.dart
diff --git a/pkg/compiler/lib/src/dart_types.dart b/pkg/compiler/lib/src/dart_types.dart
index 713cbc1414d0e4859fe0fa6d08bb3a2e92da74eb..fd27d881abab676d98fabe45ad37567e05214ade 100644
--- a/pkg/compiler/lib/src/dart_types.dart
+++ b/pkg/compiler/lib/src/dart_types.dart
@@ -475,7 +475,7 @@ class InterfaceType extends GenericType {
    * Returns the type as an instance of class [other], if possible, null
    * otherwise.
    */
-  DartType asInstanceOf(ClassElement other) {
+  InterfaceType asInstanceOf(ClassElement other) {
     other = other.declaration;
     if (element == other) return this;
     InterfaceType supertype = element.asInstanceOf(other);