[ddc] Add empty link methods to new ddc module format

The link methods will contain all operations required to link class
hierarchies across library boundaries.

Change-Id: Iaa2bc566e17d215fd8bd9a20e6fc668679a2b58c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/380208
Reviewed-by: Mark Zhou <[email protected]>
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Mark Zhou <[email protected]>

pkg/dev_compiler/lib/js/ddc/ddc_module_loader.js

@@ -1325,8 +1325,9 @@ if (!self.deferred_loader) {
           throw 'Library not defined: ' + libraryName + '. Failed to initialize.';
         }
         currentLibrary = initializer(currentLibrary);
-        // TODO(nshahan): Link the library when link methods are added to the
-        // compiled output.
+        // Link the library. This action will trigger the initialization and
+        // linking of dependency libraries as needed.
+        currentLibrary.link();
         this.libraries[libraryName] = currentLibrary;
       }
       if (installFn != null) {

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

@@ -915,11 +915,33 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
       _emitLibraryProcedures(library);
       _emitTopLevelFields(library.fields);
     }
-
+    if (_options.emitLibraryBundle) {
+      // TODO(nshahan): Remove when the Dart SDK can be compiled with the
+      // `LibraryBundleCompiler`.
+      _moduleItems.add(_emitEmptyLinkMethod(
+          _jsLibraryName(library), _emitLibraryName(library)));
+    }
     _staticTypeContext.leaveLibrary(_currentLibrary!);
     _currentLibrary = null;
   }
 
+  /// Returns an empty placeholder link method for the libraries in the SDK.
+  ///
+  /// This is a temporary solution to allow the Dart SDK to act like it was
+  /// compiled as a bundle of individual libraries.
+  // TODO(nshahan): Remove when the Dart SDK can be compiled with the
+  // `LibraryBundleCompiler`.
+  js_ast.Statement _emitEmptyLinkMethod(
+      String libraryName, js_ast.Identifier libraryId) {
+    assert(_options.emitLibraryBundle && _isBuildingSdk);
+    var functionName = _emitTemporaryId('link__$libraryName');
+    return js.statement('# = #', [
+      js_ast.PropertyAccess.field(libraryId, 'link'),
+      js_ast.NamedFunction(
+          functionName, js_ast.Fun(const [], js_ast.Block(const [])))
+    ]);
+  }
+
   void _emitExports(Library library) {
     assert(_currentLibrary == null);
     _currentLibrary = library;
@@ -8242,6 +8264,15 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
   /// field of the result.
   js_ast.Program _finishModule(List<js_ast.ModuleItem> items, String moduleName,
       {List<js_ast.Comment> header = const []}) {
+    if (_options.emitLibraryBundle) {
+      assert(_isBuildingSdk);
+      // Manually add a link method for the runtime "dartx" library. It is
+      // synthetically created by DDC and doesn't have an associated kernel
+      // library node.
+      // TODO(nshahan): Remove when the Dart SDK can be compiled with the
+      // `LibraryBundleCompiler`.
+      _moduleItems.add(_emitEmptyLinkMethod('dartx', _extensionSymbolsModule));
+    }
     // TODO(jmesserly): there's probably further consolidation we can do
     // between DDC's two backends, by moving more code into this method, as the
     // code between `startModule` and `finishModule` is very similar in both.

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

@@ -877,6 +877,29 @@ class LibraryCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     return compiledLibrary;
   }
 
+  /// Returns a method that will perform all class hierarchy operations for the
+  /// classes defined in this module.
+  ///
+  /// At a high level this method performs the prototype stitching for all
+  /// `class A extends B` relationships but in practice will also include the
+  /// operations that implicitly depend on those relationships to be established
+  /// so they can walk the prototype chain.
+  js_ast.Statement _emitLibraryLinkMethod(Library library) {
+    var libraryName = _emitLibraryName(library);
+    var nameExpr = js_ast.PropertyAccess.field(libraryName, 'link');
+    var functionName = _emitTemporaryId('link__${_jsLibraryName(library)}');
+
+    var parameters = const <js_ast.Parameter>[];
+    var body = js_ast.Block([
+      // TODO(nshahan): Remove logging and add linking statements here.
+      js.statement(
+          'console.log("Linking library: ${_jsLibraryName(library)}")'),
+    ]);
+    var function =
+        js_ast.NamedFunction(functionName, js_ast.Fun(parameters, body));
+    return js.statement('# = #', [nameExpr, function]);
+  }
+
   /// Choose a canonical name from the [library] element.
   String _jsLibraryName(Library library) {
     return libraryUriToJsIdentifier(library.importUri);
@@ -998,7 +1021,8 @@ class LibraryCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
       _emitLibraryProcedures(library);
       _emitTopLevelFields(library.fields);
     }
-
+    // Additional method used by the module system to link class hierarchies.
+    _moduleItems.add(_emitLibraryLinkMethod(library));
     _staticTypeContext.leaveLibrary(_currentLibrary!);
   }