Store the async-await compiled pub code directly in the repo.

sdk/lib/_internal/pub_generated/test/build/cleans_entire_build_directory_test.dart

+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration("cleans entire build directory before a build", () {
+    d.dir(
+        appPath,
+        [
+            d.appPubspec(),
+            d.dir('example', [d.file('file.txt', 'example')]),
+            d.dir('test', [d.file('file.txt', 'test')])]).create();
+    schedulePub(
+        args: ["build", "example"],
+        output: new RegExp(r'Built 1 file to "build".'));
+    schedulePub(
+        args: ["build", "test"],
+        output: new RegExp(r'Built 1 file to "build".'));
+    d.dir(
+        appPath,
+        [
+            d.dir(
+                'build',
+                [
+                    d.nothing('example'),
+                    d.dir('test', [d.file('file.txt', 'test')])])]).validate();
+  });
+}