Initial --modules=es6 support

lib/src/codegen/module_builder.dart

Index: lib/src/codegen/module_builder.dart
diff --git a/lib/src/codegen/module_builder.dart b/lib/src/codegen/module_builder.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f3c073c78134b50912ab634781f55a5013ced723
--- /dev/null
+++ b/lib/src/codegen/module_builder.dart
@@ -0,0 +1,138 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dev_compiler.src.codegen.js_codegen;
+
+import '../js/js_ast.dart' as JS;
+import '../js/js_ast.dart' show js;
+import '../options.dart' show ModuleFormat;
+
+final _builderCtors = {
+  ModuleFormat.es6: new ES6ModuleBuilder#,
+  ModuleFormat.dart: new DartModuleBuilder#
+};
+
+abstract class ModuleBuilder {
+  final String _jsPath;
+  final String _jsModuleValue;
+  final JS.Identifier _exportsVar;
+  final List<String> _exports = <String>[];
+  final List<_ModuleImport> _imports = <_ModuleImport>[];
+
+  ModuleBuilder._(this._jsPath, this._jsModuleValue, this._exportsVar);
+
+  factory ModuleBuilder(String jsPath, String _jsModuleValue, JS.Identifier _exportsVar, ModuleFormat format) {
+    var ctor = _builderCtors[format];
+    if (ctor == null) throw new ArgumentError("Unsupported format: $format");
+    return ctor(jsPath, _jsModuleValue, _exportsVar);
+  }
+
+  void addExport(String name) {
+    _exports.add(name);
+  }
+  void addImport(String name, JS.Identifier libVar, {bool isLazy: false}) {
+    _imports.add(new _ModuleImport(name, libVar, isLazy));
+  }
+
+  List<JS.ModuleItem> build(List<JS.Statement> moduleItems);
+}
+
+class _ModuleImport {
+  final String name;
+  final JS.Identifier libVar;
+  final bool isLazy;
+  _ModuleImport(this.name, this.libVar, this.isLazy);
+}
+
+/// Generates modules for with DDC's `dart_library.js` loading mechanism.
+class DartModuleBuilder extends ModuleBuilder {
+
+  DartModuleBuilder(jsPath, _jsModuleValue, _exportsVar)
+      : super._(jsPath, _jsModuleValue, _exportsVar);
+
+  List<JS.ModuleItem> build(List<JS.Statement> moduleItems) {
+    // TODO(jmesserly): it would be great to run the renamer on the body,
+    // then figure out if we really need each of these parameters.
+    // See ES6 modules: https://github.com/dart-lang/dev_compiler/issues/34
+    var params = [_exportsVar];
+    var lazyParams = [];
+
+    var imports = <JS.Expression>[];
+    var lazyImports = <JS.Expression>[];
+    var moduleStatements = <JS.Statement>[];
+
+    for (var i in _imports) {
+      (i.isLazy ? lazyImports : imports).add(js.string(i.name, "'"));
+      (i.isLazy ? lazyParams : params).add(i.libVar);
+    }
+    params.addAll(lazyParams);
+
+    moduleStatements.addAll(_flattenBlocks(moduleItems));
+
+    if (_exports.isNotEmpty) {
+      moduleStatements.add(js.comment('Exports:'));
+      // TODO(jmesserly): make these immutable in JS?
+      for (var name in _exports) {
+        moduleStatements.add(js.statement('#.# = #;', [_exportsVar, name, name]));
+      }
+    }
+
+    var module =
+        js.call("function(#) { 'use strict'; #; }", [params, moduleStatements]);
+
+    var moduleDef = js.statement("dart_library.library(#, #, #, #, #)", [
+      js.string(_jsPath, "'"),
+      _jsModuleValue ?? new JS.LiteralNull(),
+      js.commentExpression(
+          "Imports", new JS.ArrayInitializer(imports, multiline: true)),
+      js.commentExpression("Lazy imports",
+          new JS.ArrayInitializer(lazyImports, multiline: true)),
+      module
+    ]);
+    return <JS.ModuleItem>[moduleDef];
+  }
+}
+
+/// Generates ES6 modules.
+///
+/// TODO(ochafik): Break strong dep cycles to accommodate the Closure Compiler.
+class ES6ModuleBuilder extends ModuleBuilder {
+
+  ES6ModuleBuilder(jsPath, _jsModuleValue, _exportsVar)
+      : super._(jsPath, _jsModuleValue, _exportsVar);
+
+  List<JS.ModuleItem> build(List<JS.Statement> moduleItems) {
+    // TODO(jmesserly): it would be great to run the renamer on the body,
+    // then figure out if we really need each of these parameters.
+    // See ES6 modules: https://github.com/dart-lang/dev_compiler/issues/34
+    var moduleStatements = <JS.ModuleItem>[];
+
+    moduleStatements.add(js.statement("'use strict';"));
+    // Lazy declarations may reference exports.
+    moduleStatements.add(js.statement("const # = {};", [_exportsVar]));
+
+    for (var i in _imports) {
+      // TODO(ochafik): laziness, late binding, etc, to support Closure...
+      moduleStatements.add(new JS.ImportDeclaration(
+          defaultBinding: i.libVar,
+          from: js.string(i.name)));
+    }
+
+    moduleStatements.addAll(_flattenBlocks(moduleItems));
+
+    if (_exports.isNotEmpty) {
+      moduleStatements.add(js.comment('Exports:'));
+      // TODO(jmesserly): make these immutable in JS?
+      for (var name in _exports) {
+        moduleStatements.add(js.statement('#.# = #;', [_exportsVar, name, name]));
+      }
+      moduleStatements.add(new JS.ExportDeclaration(_exportsVar, isDefault: true));
+    }
+    // TODO(ochafik): What to do of _jsModuleValue?
+    return moduleStatements;
+  }
+}
+
+Iterable<JS.ModuleItem> _flattenBlocks(List<JS.ModuleItem> stats) =>
+    stats.expand((item) => item is JS.Block ?  item.statements : [item]);