Deferred parsing for legacy modules

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

Index: pkg/dev_compiler/lib/src/compiler/module_builder.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/module_builder.dart b/pkg/dev_compiler/lib/src/compiler/module_builder.dart
index 415554a81ce7b21bda89328d94a4be835a314c55..355f3effaf5c6f9e2c91b956a79e6606db96da2f 100644
--- a/pkg/dev_compiler/lib/src/compiler/module_builder.dart
+++ b/pkg/dev_compiler/lib/src/compiler/module_builder.dart
@@ -161,8 +161,19 @@ class LegacyModuleBuilder extends _ModuleBuilder {
       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);