Fix a couple bugs in r42055.

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

Index: sdk/lib/_internal/pub_generated/bin/async_compile.dart
diff --git a/sdk/lib/_internal/pub_generated/bin/async_compile.dart b/sdk/lib/_internal/pub_generated/bin/async_compile.dart
index ffdd761d4f4bf9949309312f0b5cf4095dbd2faf..a029c521294cef1603afbae148655e364cf1c8c6 100644
--- a/sdk/lib/_internal/pub_generated/bin/async_compile.dart
+++ b/sdk/lib/_internal/pub_generated/bin/async_compile.dart
@@ -61,9 +61,10 @@ void main(List<String> arguments) {
 
     buildDir = rest.first;
   } on FormatException catch (ex) {
-    print(ex);
-    print();
-    print("Usage: dart async_compile.dart [--verbose] [--force] <build dir>");
+    stderr.writeln(ex);
+    stderr.writeln();
+    stderr.writeln(
+        "Usage: dart async_compile.dart [--verbose] [--force] <build dir>");
     exit(64);
   }
 
@@ -77,7 +78,7 @@ void main(List<String> arguments) {
   var readme = new File(readmePath).readAsStringSync();
   var match = _commitPattern.firstMatch(readme);
   if (match == null) {
-    print("Could not find compiler commit hash in README.md.");
+    stderr.writeln("Could not find compiler commit hash in README.md.");
     exit(1);
   }
 
@@ -150,7 +151,7 @@ String _getCurrentCommit() {
       args,
       workingDirectory: p.join(sourceDir, "../../../../third_party/pkg/async_await"));
   if (result.exitCode != 0) {
-    print("Could not get Git revision of async_await compiler.");
+    stderr.writeln("Could not get Git revision of async_await compiler.");
     exit(1);
   }
 
@@ -192,7 +193,7 @@ String _translateAsyncAwait(String sourcePath, String source) {
     var result = new CodeFormatter().format(CodeKind.COMPILATION_UNIT, source);
     return result.source;
   } catch (ex) {
-    print("Async compile failed on $sourcePath:\n$ex");
+    stderr.writeln("Async compile failed on $sourcePath:\n$ex");
     hadFailure = true;
     return null;
   }
@@ -225,9 +226,9 @@ void _generateSnapshot(String buildDir) {
       ["--package-root=$packageRoot", "--snapshot=$snapshot", entrypoint]);
 
   if (result.exitCode != 0) {
-    print("Failed to generate snapshot:");
-    if (result.stderr.trim().isNotEmpty) print(result.stderr);
-    if (result.stdout.trim().isNotEmpty) print(result.stdout);
+    stderr.writeln("Failed to generate snapshot:");
+    if (result.stderr.trim().isNotEmpty) stderr.writeln(result.stderr);
+    if (result.stdout.trim().isNotEmpty) stderr.writeln(result.stdout);
     exit(result.exitCode);
   }