Properly handle `ProcessExit` being thrown when `exit()` is called (#8592)

packages/flutter_tools/lib/executable.dart

@@ -182,7 +182,12 @@ Future<int> _handleToolError(
     return _exit(error.exitCode ?? 1);
   } else if (error is ProcessExit) {
     // We've caught an exit code.
-    return _exit(error.exitCode);
+    if (error.immediate) {
+      exit(error.exitCode);
+      return error.exitCode;
+    } else {
+      return _exit(error.exitCode);
+    }
   } else {
     // We've crashed; emit a log report.
     stderr.writeln();
@@ -217,7 +222,11 @@ Future<int> _handleToolError(
             'Unable to generate crash report due to secondary error: $error\n'
                 'please let us know at https://github.com/flutter/flutter/issues.',
         );
-        return _exit(1);
+        // Any exception throw here (including one thrown by `_exit()`) will
+        // get caught by our zone's `onError` handler. In order to avoid an
+        // infinite error loop, we throw an error that is recognized above
+        // and will trigger an immediate exit.
+        throw new ProcessExit(1, immediate: true);
       }
     }
   }
@@ -299,9 +308,13 @@ Future<int> _exit(int code) async {
 
   // Give the task / timer queue one cycle through before we hard exit.
   Timer.run(() {
-    printTrace('exiting with code $code');
-    exit(code);
-    completer.complete();
+    try {
+      printTrace('exiting with code $code');
+      exit(code);
+      completer.complete();
+    } catch (error, stackTrace) {
+      completer.completeError(error, stackTrace);
+    }
   });
 
   await completer.future;

packages/flutter_tools/lib/src/base/io.dart

@@ -25,10 +25,12 @@
 /// about any additional exports that you add to this file, as doing so will
 /// increase the API surface that we have to test in Flutter tools, and the APIs
 /// in `dart:io` can sometimes be hard to use in tests.
-import 'dart:io' as io show exit, exitCode;
+import 'dart:io' as io show exit;
 
 import 'package:meta/meta.dart';
 
+import 'process.dart';
+
 export 'dart:io'
     show
         BytesBuilder,
@@ -88,7 +90,7 @@ ExitFunction get exit => _exitFunction;
 @visibleForTesting
 void setExitFunctionForTests([ExitFunction exitFunction]) {
   _exitFunction = exitFunction ?? (int exitCode) {
-    throw new Exception('Exited with code ${io.exitCode}');
+    throw new ProcessExit(exitCode, immediate: true);
   };
 }
 

packages/flutter_tools/lib/src/base/process.dart

@@ -295,8 +295,9 @@ String _runWithLoggingSync(List<String> cmd, {
 }
 
 class ProcessExit implements Exception {
-  ProcessExit(this.exitCode);
+  ProcessExit(this.exitCode, {this.immediate: false});
 
+  final bool immediate;
   final int exitCode;
 
   String get message => 'ProcessExit: $exitCode';