Move helpers and interceptors to own library.

dart/frog/leg/typechecker.dart

Index: dart/frog/leg/typechecker.dart
diff --git a/dart/frog/leg/typechecker.dart b/dart/frog/leg/typechecker.dart
index fd92d05796f90c4632b9d7ffbdb2693ad146e6c1..db4df503ea0f19cd4eb389d2dac58664ff90be25 100644
--- a/dart/frog/leg/typechecker.dart
+++ b/dart/frog/leg/typechecker.dart
@@ -375,7 +375,9 @@ class TypeCheckerVisitor implements Visitor<Type> {
         funType = memberType;
       } else {
         Element element = elements[node];
-        if (element.kind === ElementKind.FUNCTION) {
+        if (element === null) {
+          fail(node, 'unresolved ${node.selector}');
+        } else if (element.kind === ElementKind.FUNCTION) {
           funType = computeType(element);
         } else if (element.kind === ElementKind.FOREIGN) {
           return types.dynamicType;