Revert "Use native async/await support in pub."

sdk/lib/_internal/pub_generated/README.md

Index: sdk/lib/_internal/pub_generated/README.md
diff --git a/sdk/lib/_internal/pub_generated/README.md b/sdk/lib/_internal/pub_generated/README.md
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index 0000000000000000000000000000000000000000..903a085c0ea22528f5e117aef36e4663af296fec
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+++ b/sdk/lib/_internal/pub_generated/README.md
@@ -0,0 +1,18 @@
+Pub is currently dogfooding the new Dart async/await syntax. Since the Dart VM
+doesn't natively support it yet, we are using the [async-await][] compiler
+package.
+
+[async-await]: https://github.com/dart-lang/async_await
+
+We run that to compile pub-using-await from sdk/lib/_internal/pub down to
+vanilla Dart code which is what you see here. To interoperate more easily with
+the rest of the repositry, we check in that generated code.
+
+When bug #104 is fixed, we can remove this entirely.
+
+The code here was compiled using the async-await compiler at commit:
+
+    8b401a9f2e5e81dca5f70dbe7564112a0823dee6
+
+(Note: this file is also parsed by a tool to update the above commit, so be
+careful not to reformat it.)