[ddc] Adding a runtime flag for real deferred loading in DDC

Change-Id: If50f7437c827a32829f5093cb3a14cbc5e55f7a7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289340
Commit-Queue: Mark Zhou <markzipan@google.com>
Reviewed-by: Anna Gringauze <annagrin@google.com>

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -608,16 +608,25 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
   }
 
   @override
-  String libraryToModule(Library library) {
+  String libraryToModule(Library library, {bool throwIfNotFound = true}) {
     if (library.importUri.isScheme('dart')) {
       // TODO(jmesserly): we need to split out HTML.
       return js_ast.dartSdkModule;
     }
-    var summary = _importToSummary[library]!;
+    var summary = _importToSummary[library];
+    if (summary == null) {
+      if (throwIfNotFound) {
+        throw StateError('Could not find summary for library "$library".');
+      }
+      return '';
+    }
     var moduleName = _summaryToModule[summary];
     if (moduleName == null) {
-      throw StateError('Could not find module name for library "$library" '
-          'from component "$summary".');
+      if (throwIfNotFound) {
+        throw StateError('Could not find module name for library "$library" '
+            'from component "$summary".');
+      }
+      return '';
     }
     return moduleName;
   }
@@ -7050,9 +7059,11 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
 
   @override
   js_ast.Expression visitLoadLibrary(LoadLibrary node) =>
-      runtimeCall('loadLibrary(#, #)', [
+      runtimeCall('loadLibrary(#, #, #)', [
         js.string(node.import.enclosingLibrary.importUri.toString()),
-        js.string(node.import.name!)
+        js.string(node.import.name!),
+        js.string(
+            libraryToModule(node.import.targetLibrary, throwIfNotFound: false))
       ]);
 
   // TODO(jmesserly): DDC loads all libraries eagerly.

sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart

@@ -856,32 +856,51 @@ _canonicalMember(obj, name) {
   return name;
 }
 
+@notNull
+bool _realDeferredLoading = false;
+
+/// Sets the runtime mode to perform deferred loading (instead of just runtime
+/// correctness checks on loaded libraries).
+///
+/// This is only supported in the DDC module system.
+void realDeferredLoading(bool enable) {
+  _realDeferredLoading = enable;
+}
+
 /// A map from libraries to a set of import prefixes that have been loaded.
 ///
 /// Used to validate deferred library conventions.
 final deferredImports = JS<Object>('!', 'new Map()');
 
-/// Emulates the implicit "loadLibrary" function provided by a deferred library.
+/// Loads the element [importPrefix] in the module [targetModule] from the
+/// context of the library [libraryUri].
 ///
-/// Libraries are not actually deferred in DDC, so this just records the import
-/// for runtime validation, then returns a future that completes immediately.
-Future loadLibrary(
-    @notNull String enclosingLibrary, @notNull String importPrefix) {
-  var result = JS('', '#.get(#)', deferredImports, enclosingLibrary);
-  if (JS<bool>('', '# === void 0', result)) {
-    JS('', '#.set(#, # = new Set())', deferredImports, enclosingLibrary,
-        result);
+/// Will load any modules required by [targetModule] as a side effect.
+/// Only supported in the DDC module system.
+Future<void> loadLibrary(@notNull String libraryUri,
+    @notNull String importPrefix, @notNull String targetModule) {
+  if (!_realDeferredLoading) {
+    var result = JS('', '#.get(#)', deferredImports, libraryUri);
+    if (JS<bool>('', '# === void 0', result)) {
+      JS('', '#.set(#, # = new Set())', deferredImports, libraryUri, result);
+    }
+    JS('', '#.add(#)', result, importPrefix);
+    return Future.value();
+  } else {
+    throw UnimplementedError('realDeferredLoading flag is not yet supported');
   }
-  JS('', '#.add(#)', result, importPrefix);
-  return Future.value();
 }
 
 void checkDeferredIsLoaded(
-    @notNull String enclosingLibrary, @notNull String importPrefix) {
-  var loaded = JS('', '#.get(#)', deferredImports, enclosingLibrary);
-  if (JS<bool>('', '# === void 0', loaded) ||
-      JS<bool>('', '!#.has(#)', loaded, importPrefix)) {
-    throwDeferredIsLoadedError(enclosingLibrary, importPrefix);
+    @notNull String libraryUri, @notNull String importPrefix) {
+  if (!_realDeferredLoading) {
+    var loaded = JS('', '#.get(#)', deferredImports, libraryUri);
+    if (JS<bool>('', '# === void 0', loaded) ||
+        JS<bool>('', '!#.has(#)', loaded, importPrefix)) {
+      throwDeferredIsLoadedError(libraryUri, importPrefix);
+    }
+  } else {
+    throw UnimplementedError('realDeferredLoading flag is not yet supported');
   }
 }