fix(kernel): Improve tagged type of wire values

When passing references across the JSII language boundary, the static
return type of a method is used instead of the runtime type of the
object if they are not the same, even if they are compatible. This
causes issues that make it impossible to up-cast from methods such as
`IConstruct.findChild` that return a super-class that is expected to be
up-casted to it's known dynamic type.

Fixes #345

packages/jsii-kernel/lib/kernel.ts

@@ -961,7 +961,7 @@ export class Kernel {
             // have an object id, so we need to allocate one for it.
             this._debug('creating objref for', v);
             const fqn = this._fqnForObject(v);
-            if (!targetType || !spec.isNamedTypeReference(targetType) || fqn === targetType.fqn) {
+            if (!targetType || !spec.isNamedTypeReference(targetType) || this._isAssignable(fqn, targetType)) {
                 return this._createObjref(v, fqn);
             }
         }
@@ -1029,6 +1029,31 @@ export class Kernel {
         return v;
     }
 
+    /**
+     * Tests whether a given type (by it's FQN) can be assigned to a named type reference.
+     *
+     * @param actualTypeFqn the FQN of the type that is being tested.
+     * @param requiredType  the required reference type.
+     *
+     * @returns true if ``requiredType`` is a super-type (base class or implemented interface) of the type designated by
+     *          ``actualTypeFqn``.
+     */
+    private _isAssignable(actualTypeFqn: string, requiredType: spec.NamedTypeReference): boolean {
+        if (requiredType.fqn === actualTypeFqn) {
+            return true;
+        }
+        const actualType = this._typeInfoForFqn(actualTypeFqn);
+        if (spec.isClassType(actualType) && actualType.base) {
+            if (this._isAssignable(actualType.base.fqn, requiredType)) {
+                return true;
+            }
+        }
+        if (spec.isClassOrInterfaceType(actualType) && actualType.interfaces) {
+            return actualType.interfaces.find(iface => this._isAssignable(iface.fqn, requiredType)) != null;
+        }
+        return false;
+    }
+
     private _toSandboxValues(args: any[]) {
         return args.map(v => this._toSandbox(v));
     }