make sure to init instances extensions can use (ampproject#5161)

* make sure to init instances extensions can use

* add comments on why we do this

3p/integration.js

@@ -104,8 +104,6 @@ import {yieldone} from '../ads/yieldone';
 import {zergnet} from '../ads/zergnet';
 import {zucks} from '../ads/zucks';
 
-initLogConstructor();
-
 /**
  * Whether the embed type may be used with amp-embed tag.
  * @const {!Object<string, boolean>}
@@ -121,6 +119,9 @@ const AMP_EMBED_ALLOWED = {
 const data = parseFragment(location.hash);
 window.context = data._context;
 
+// This should only be invoked after window.context is set
+initLogConstructor();
+
 if (getMode().test || getMode().localDev) {
   register('_ping_', _ping_);
 }

src/log.js

@@ -422,6 +422,17 @@ let logConstructor = null;
 
 export function initLogConstructor() {
   logConstructor = Log;
+  // Initialize instances for use. If a binary (an extension for example) that
+  // does not call `initLogConstructor` invokes `dev()` or `user()` earlier
+  // than the binary that does call `initLogConstructor` (amp.js), the extension
+  // will throw an error as that extension will never be able to initialize
+  // the log instances and we also don't want it to call `initLogConstructor`
+  // either (since that will cause the Log implementation to be bundled into that
+  // binary). So we must initialize the instances eagerly so that they are
+  // ready for use (stored globally) after the main binary calls
+  // `initLogConstructor`.
+  dev();
+  user();
 }
 
 export function resetLogConstructorForTesting() {

src/runtime.js

@@ -27,7 +27,7 @@ import {
 } from './service/extensions-impl';
 import {ampdocServiceFor} from './ampdoc';
 import {cssText} from '../build/css';
-import {dev, user} from './log';
+import {dev, user, initLogConstructor} from './log';
 import {fromClassForDoc, getService, getServiceForDoc} from './service';
 import {childElementsByTag} from './dom';
 import {
@@ -59,7 +59,6 @@ import {installViewportService} from './service/viewport-impl';
 import {installVsyncService} from './service/vsync-impl';
 import {installXhrService} from './service/xhr-impl';
 import {isExperimentOn, toggleExperiment} from './experiments';
-import {initLogConstructor} from './log';
 import {platformFor} from './platform';
 import {registerElement} from './custom-element';
 import {registerExtendedElement} from './extended-element';