Use precompiled dependencies in pub.

sdk/lib/_internal/pub_generated/bin/async_compile.dart

 import 'dart:io';
 import 'package:args/args.dart';
 import 'package:analyzer/src/services/formatter_impl.dart';
 import 'package:async_await/async_await.dart' as async_await;
 import 'package:path/path.dart' as p;
 final sourceDir = p.dirname(p.dirname(p.fromUri(Platform.script)));
 final sourceUrl = p.toUri(sourceDir).toString();
 final generatedDir = p.join(p.dirname(sourceDir), 'pub_generated');
 bool hadFailure = false;
 bool verbose = false;
@@ skipping 34 matching lines @@
   var lastCommit;
   var readme = new File(readmePath).readAsStringSync();
   var match = _commitPattern.firstMatch(readme);
   if (match == null) {
     stderr.writeln("Could not find compiler commit hash in README.md.");
     exit(1);
   }
   lastCommit = match[0];
   var numFiles = 0;
   var numCompiled = 0;
-  var sources = new Set<String>();
+  var sources = new Set();
   for (var entry in new Directory(sourceDir).listSync(recursive: true)) {
     if (p.extension(entry.path) != ".dart") continue;
     numFiles++;
     var relative = p.relative(entry.path, from: sourceDir);
     sources.add(relative);
     var sourceFile = entry as File;
     var destPath = p.join(generatedDir, relative);
     var destFile = new File(destPath);
     if (force ||
         currentCommit != lastCommit ||
@@ skipping 71 matching lines @@
 void _deleteFile(String path) {
   try {
     new File(path).deleteSync();
   } on IOException catch (ex) {}
 }
 void _writeFile(String path, String contents) {
   try {
     new File(path).writeAsStringSync(contents);
   } on IOException catch (ex) {}
 }