[ES6 modules] Implement https://html.spec.whatwg.org/#resolve-a-module-specifier

third_party/WebKit/Source/core/dom/Modulator.cpp

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/Modulator.h"
 
 #include "bindings/core/v8/ScriptState.h"
 
 namespace blink {
 
 Modulator* Modulator::from(LocalFrame* frame) {
   ScriptState* scriptState = ScriptState::forMainWorld(frame);
   if (!scriptState)
     return nullptr;
   // TODO(kouhei): setModulator in V8PerContextData when we land ModulatorImpl.
   return scriptState->perContextData()->modulator();
 }
 
+KURL Modulator::resolveModuleSpecifier(const String& moduleRequest,
+                                       const KURL& baseURL) {
+  // Step 1. Apply the URL parser to specifier. If the result is not failure,
+  // return the result.
+  KURL url(KURL(), moduleRequest);
+  if (url.isValid())
+    return url;
+
+  // Step 2. If specifier does not start with the character U+002F SOLIDUS (/),
+  // the two-character sequence U+002E FULL STOP, U+002F SOLIDUS (./), or the
+  // three-character sequence U+002E FULL STOP, U+002E FULL STOP, U+002F SOLIDUS
+  // (../), return failure and abort these steps.
+  if (!moduleRequest.startsWith("/") && !moduleRequest.startsWith("./") &&
+      !moduleRequest.startsWith("../"))
+    return KURL();
+
+  // Step 3. Return the result of applying the URL parser to specifier with
+  // script's base URL as the base URL.
+  DCHECK(baseURL.isValid());
+  KURL absoluteURL(baseURL, moduleRequest);
+  if (absoluteURL.isValid())
+    return absoluteURL;
+
+  return KURL();
+}
+
 }  // namespace blink