Add case for custom toprimitive operations

test/language/expressions/dynamic-import/custom-primitive.js

+// Copyright (C) 2018 Leo Balter. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+description: >
+    Import a custom toString and valueOf bindings
+esid: sec-finishdynamicimport
+info: |
+    Runtime Semantics: FinishDynamicImport ( referencingScriptOrModule, specifier, promiseCapability, completion )
+
+    2. Otherwise,
+        a. Assert: completion is a normal completion and completion.[[Value]] is undefined.
+        b. Let moduleRecord be ! HostResolveImportedModule(referencingScriptOrModule, specifier).
+        c. Assert: Evaluate has already been invoked on moduleRecord and successfully completed.
+        d. Let namespace be GetModuleNamespace(moduleRecord).
+        ...
+        f. Otherwise, perform ! Call(promiseCapability.[[Resolve]], undefined, « namespace.[[Value]] »).
+flags: [async]
+features: [dynamic-import]
+---*/
+
+async function fn() {
+    const str = await import('./custom-tostring_FIXTURE.js');
+    const value = await import('./custom-valueof_FIXTURE.js');
+
+    assert.sameValue(String(str), '1612', 'namespace uses the imported toString');
+    assert.sameValue(Number(str), 1612, 'namespace fallsback to toString as its prototype is null');
+
+    assert.sameValue(Number(value), 42, 'namespace uses the imported valueOf');
+    assert.sameValue(String(value), '42', 'namespace fallsback to valueOf as its prototype is null');
+}
+
+fn().then($DONE, $DONE);

test/language/expressions/dynamic-import/custom-tostring_FIXTURE.js

+// Copyright (C) 2018 Leo Balter. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+export function toString() {
+    return '1612';
+}

test/language/expressions/dynamic-import/custom-valueof_FIXTURE.js

+// Copyright (C) 2018 Leo Balter. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+export function valueOf() {
+    return 42;
+}