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

sdk/lib/_internal/pub_generated/lib/src/preprocess.dart

+library pub.preprocess;
+import 'package:string_scanner/string_scanner.dart';
+import 'version.dart';
+String preprocess(String input, Map<String, Version> versions, sourceUrl) {
+  if (!input.contains(new RegExp(r"^//[>#]", multiLine: true))) return input;
+  return new _Preprocessor(input, versions, sourceUrl).run();
+}
+class _Preprocessor {
+  final StringScanner _scanner;
+  final Map<String, Version> _versions;
+  final _buffer = new StringBuffer();
+  _Preprocessor(String input, this._versions, sourceUrl)
+      : _scanner = new StringScanner(input, sourceUrl: sourceUrl);
+  String run() {
+    while (!_scanner.isDone) {
+      if (_scanner.scan(new RegExp(r"//#[ \t]*"))) {
+        _if();
+      } else {
+        _emitText();
+      }
+    }
+    _scanner.expectDone();
+    return _buffer.toString();
+  }
+  void _emitText() {
+    while (!_scanner.isDone && !_scanner.matches("//#")) {
+      if (_scanner.scan("//>")) {
+        if (!_scanner.matches("\n")) _scanner.expect(" ");
+      }
+      _scanner.scan(new RegExp(r"[^\n]*\n?"));
+      _buffer.write(_scanner.lastMatch[0]);
+    }
+  }
+  void _ignoreText() {
+    while (!_scanner.isDone && !_scanner.matches("//#")) {
+      _scanner.scan(new RegExp(r"[^\n]*\n?"));
+    }
+  }
+  void _if() {
+    _scanner.expect(new RegExp(r"if[ \t]+"), name: "if statement");
+    _scanner.expect(new RegExp(r"[a-zA-Z0-9_]+"), name: "package name");
+    var package = _scanner.lastMatch[0];
+    _scanner.scan(new RegExp(r"[ \t]*"));
+    var constraint = VersionConstraint.any;
+    if (_scanner.scan(new RegExp(r"[^\n]+"))) {
+      try {
+        constraint = new VersionConstraint.parse(_scanner.lastMatch[0]);
+      } on FormatException catch (error) {
+        _scanner.error("Invalid version constraint: ${error.message}");
+      }
+    }
+    _scanner.expect("\n");
+    var allowed =
+        _versions.containsKey(package) &&
+        constraint.allows(_versions[package]);
+    if (allowed) {
+      _emitText();
+    } else {
+      _ignoreText();
+    }
+    _scanner.expect("//#");
+    _scanner.scan(new RegExp(r"[ \t]*"));
+    if (_scanner.scan("else")) {
+      _scanner.expect("\n");
+      if (allowed) {
+        _ignoreText();
+      } else {
+        _emitText();
+      }
+      _scanner.expect("//#");
+      _scanner.scan(new RegExp(r"[ \t]*"));
+    }
+    _scanner.expect("end");
+    if (!_scanner.isDone) _scanner.expect("\n");
+  }
+}