Deferred parsing for legacy modules

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

 // 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.
 
 import 'package:args/args.dart' show ArgParser, ArgResults;
 import 'package:path/path.dart' as path;
 
 import '../js_ast/js_ast.dart';
 import 'js_names.dart';
 
@@ skipping 124 matching lines @@
     statements.add(node.exported.toStatement());
   }
 
   visitStatement(Statement node) {
     statements.add(node);
   }
 }
 
 /// Generates modules for with our legacy `dart_library.js` loading mechanism.
 // TODO(jmesserly): remove this and replace with something that interoperates.
 class LegacyModuleBuilder extends _ModuleBuilder {

[Jennifer Messerly, 2017/02/15 20:08:49]

Should we rename this, DeferredModuleBuilder, and change the terminology from
"legal" to "defer" in help and options and docs? 

It seems like this isn't legacy anymore, but a format that is supported because
it offers a unique feature (defer support).

Otherwise, do we intend to offer defer support for other module formats?

   Program build(Program module) {
     // Collect imports/exports/statements.
     visitProgram(module);
 
     // Build import parameters.
     var exportsVar = new TemporaryId('exports');
     var parameters = <TemporaryId>[exportsVar];
     var importNames = <Expression>[];
     var importStatements = <Statement>[];
     for (var import in imports) {
       importNames.add(import.from);
       // TODO(jmesserly): we could use destructuring here.
       var moduleVar =
           new TemporaryId(pathToJSIdentifier(import.from.valueWithoutQuotes));
       parameters.add(moduleVar);
       for (var importName in import.namedImports) {
         assert(!importName.isStar); // import * not supported in legacy modules.
         var asName = importName.asName ?? importName.name;
-        importStatements.add(js.statement(
-            'const # = #.#', [asName, moduleVar, importName.name.name]));
+        var fromName = importName.name.name;
+        // Load non-SDK modules on demand (i.e., deferred).
+        if (import.from.valueWithoutQuotes != dartSdkModule) {
+          importStatements.add(js.statement(
+              'let # = dart_library.defer(#, #, function (mod, lib) {'

[Jennifer Messerly, 2017/02/15 20:08:49]

Does this work for a JavaScript module we import?

are we assuming that DDC won't work be able to interoperate with JavaScript
modules in the future?

[vsm, 2017/02/15 20:32:58]

If that JS module uses our legacy modules as well, it should work the same,
right?  It doesn't change interop for other module systems.  Similarly, I'm
skipping this for the SDK.

+              '  # = mod;'
+              '  # = lib;'
+              '});',
+              [asName, moduleVar, js.string(fromName), moduleVar, asName]));
+        } else {
+          importStatements.add(js.statement(
+              'const # = #.#', [asName, moduleVar, importName.name.name]));
+        }
       }
     }
     statements.insertAll(0, importStatements);
 
     if (exports.isNotEmpty) {
       statements.add(js.comment('Exports:'));
       // TODO(jmesserly): make these immutable in JS?
       for (var export in exports) {
         var names = export.exportedNames;
         assert(names != null); // export * not supported in legacy modules.
@@ skipping 158 matching lines @@
   // Ensure the identifier first character is not numeric and that the whole
   // identifier is not a keyword.
   if (result.startsWith(new RegExp('[0-9]')) || invalidVariableName(result)) {
     return '\$$result';
   }
   return result;
 }
 
 // Invalid characters for identifiers, which would need to be escaped.
 final _invalidCharInIdentifier = new RegExp(r'[^A-Za-z_$0-9]');